How do i get back my old user account after upgrading to OS X Yosemite 10.10.2?
Apple (modified a bit) helps:
This guide should/will work if your old user was the only account and the newly created user has the User ID 501.

Log out of your newly created account (user2), then log in to an admin account. The admin account needs to be a different account than the one newly created. If necessary, create another admin account (user3).

Go to the Users folder on the startup drive. The Users folder contains the home folder of the account you want to get back (user1), the newly created admin account (user3) and the newly created account while upgrading (user2).

Go to Apple menu &gt; System Preferences, then click Users &amp; Groups.

Click the lock icon in the Users &amp; Groups pane, then enter the admin name and password that you used to log in.

From the list of accounts, open a shortcut menu by Control-clicking the account that was created upgrading to Yosemite (user2). Enable the radio button &quot;Allow user to administer this computer&quot; if it is disabled. Choose &quot;Advanced Options&quot; from the shortcut menu.

Check the User ID it should be 501 (If it's not 501 stop here and add a comment to my answer please)

Change the &quot;Account name&quot; field and the &quot;Full name&quot; field to match the name of the old home folder (user1) in the Users folder and the old full name.

Change the &quot;Home directory&quot; field to match name of the old home folder (user1) in the Users folder.


Click OK, then close System Preferences.

Log out of the current admin account, then log in to the old account (now user1 again).
Verify that your old files and folders are visible and the account is working normally.

Delete the admin account (user 3) created in step 1.

Delete the user folder of user2 with sudo rm /Users/user2