You can do it in the UI, but its a little slow.
Go to Agent Panel -> Users.
Open the User you want to get rid of.
Open each and every ticket they have into a separate tab.
Edit each ticket and change the owner to the other User.
Once you have finished doing this for all tickets, delete the user.
Doing this at the DB level is probably a little faster in bulk... but there is a lot more to be concerned with.
The basics are you need to look in the ost_user table for the old users id, the new users id, and using that look in the ost_user_email. for the new users email.id.
Then you would do something like:
UPDATE ost_ticket SET user_id='17',user_email_id='17' WHERE user_id='11;
Change the first user_id='17' to the new id. (the one you want to keep)
Change the user_email_id='17' is the new email id.
The user_id='11' at the end is the old user id.
Once you've done that you have to decide if you want to muck with the ticket threads. As it stands now you could call it quits and everything will work okay. But if you want the tickets to not look like the threads were all written by someone else then you would need to do something similar for the ost_thread_entry table, ost_thread_event table and probably also the ost_thread_collaborator. You would have to take a good look at them to make sure.