Steps to migrates a website Drupal 6 to Drupal 7

First steps and definitions:

  Make a full backup of all files, directories, and your database(s) before   starting, and save it outside your Drupal installation directory.  
   It is wise to try an update or upgrade on a test copy of your site before applying it to your live site. Even minor updates can cause your site's  behavior to change. 

UPGRADE PROBLEMS

If you encounter errors during this process,

  Note any error messages you see.
  Restore your site to its previous state, using the file and database backups you created before you started the upgrade process. Do not attempt to do further upgrades on a site that had update problems. 

UPGRADE STEPS

To upgrade from a previous major version of Drupal to Drupal 7.x, after

following the instructions in the INTRODUCTION section at the top of this file:

1. Check on the Drupal 7 status of your contributed and custom modules and

   themes. See http://drupal.org/node/948216 for information on upgrading

   contributed modules and themes. See http://drupal.org/node/895314 for a list

   of modules that have been moved into core for Drupal 7, and instructions on

   how to update them. See http://drupal.org/update/modules for information on

   how to update your custom modules, and http://drupal.org/update/theme for

   custom themes. 

   You may decide at this point that you cannot upgrade your site, because

   needed modules or themes are not ready for Drupal 7.

2. Update to the latest available version of Drupal 6.x (if your current version

   is Drupal 5.x, you have to upgrade to 6.x first). If you need to update,

   download Drupal 6.x and follow the instructions in its UPGRADE.txt. This

   document only applies for upgrades from 6.x to 7.x.

3. In addition to updating to the latest available version of Drupal 6.x core,

   you must also upgrade all of your contributed modules for Drupal to their

   latest Drupal 6.x versions.

4. Log in as user ID 1 (the site maintenance user).

5. Go to Administer > Site configuration > Site maintenance. Select

   "Off-line" and save the configuration.

6. Go to Administer > Site building > Themes. Enable "Garland" and select it as

   the default theme.

7. Go to Administer > Site building > Modules. Disable all modules that are not

   listed under "Core - required" or "Core - optional". It is possible that some

   modules cannot be disabled, because others depend on them. Repeat this step

   until all non-core modules are disabled.

   If you know that you will not re-enable some modules for Drupal 7.x and you

   no longer need their data, then you can uninstall them under the Uninstall

   tab after disabling them.

8. On the command line or in your FTP client, remove the file 

     sites/default/default.settings.php

9. Remove all old core files and directories, except for the 'sites' directory

   and any custom files you added elsewhere.

   If you made modifications to files like .htaccess or robots.txt, you will

   need to re-apply them from your backup, after the new files are in place.

10. If you uninstalled any modules, remove them from the sites/all/modules and

   other sites/*/modules directories. Leave other modules in place, even though

   they are incompatible with Drupal 7.x.

11. Download the latest Drupal 7.x release from http://drupal.org to a

   directory outside of your web root. Extract the archive and copy the files

   into your Drupal directory.   

12. Re-apply any modifications to files such as .htaccess or robots.txt.

13. Make your settings.php file writeable, so that the update process can

   convert it to the format of Drupal 7.x. settings.php is usually located in

     sites/default/settings.php

14. Run update.php by visiting http://www.example.com/update.php (replace

   www.example.com with your domain name). This will update the core database

   tables. 

   If you are unable to access update.php do the following:

   - Open settings.php with a text editor.

   - Find the line that says:

     $update_free_access = FALSE;

   - Change it into:

     $update_free_access = TRUE;

   - Once the upgrade is done, $update_free_access must be reverted to FALSE.

15. Backup your database after the core upgrade has run.

16. Replace and update your non-core modules and themes, following the

   procedures at http://drupal.org/node/948216

17. Go to Administration > Reports > Status report. Verify that everything is

   working as expected.

18. Ensure that $update_free_access is FALSE in settings.php.

19. Go to Administration > Configuration > Development > Maintenance mode.

   Disable the "Put site into maintenance mode" checkbox and save the

   configuration. 

To get started with Drupal 7 administration, visit

http://drupal.org/getting-started/7/admin