The simplest way is usually just to logon as a user with administrator rights or SA and then you should be able to see the APPROVE / REJECT actions, unless you have blocking preconditions in front of these actions in your process.
Alternatively, if this is a one-off task, you need to do (ie no chance new transactions can be assigned to this user), then maybe the quickest way is to temporarily undelete the user, action the lifecycles affected (as that user) and then delete the user again. Just create a query in the console on the SYSTEM>USER object with a condition IsDeleted = true, run it and find your user, right-click and you should have an “undelete” action to restore this soft-deleted user (only works in console and not WebDesk).
If you need the Changes to be approved with audit trail by a different user, you need to change your process to temporarily allow re-assign away from the current user at whatever status(es) they are at. Maybe this feature could be permanently designed into this stagevof the process with a precondition so that only admins or Change Manager etc can action.
Possibly you could write a SQL. query to reassign in bulk too but that would not update the audit trail etc in the process which I think would be useful here.