Renaming functionality allows you to edit id of specific record. User is saved with person's email id. Only User with System Manager's role will be able to rename User IDs.
Following are the steps to rename user id.
Step 1: Users
Setup > Users > User
Open User to be renamed.
Step 2: Rename
From Menu, select Rename.
Step 3: Update
Enter valid email id and click on Rename.
After successful renaming, User will be able login using updated user id.