Instagram Access

I have a client who has ended up with 2 Instagram accounts - one is very old and has not been used for a number of years as they no longer have access to it (an old member of staff looked after it but they no longer work there so they don't have the email or password. The other one is current and active but the user name is weak. Ideally they need to regain access to the old account and then delete the new one OR delete the old one completely then set it up again with access to it as a brand new account.

@therunningmare is the old account
@the.runningmare is the new account

I hope this makes sense please ask for more info if required