A user has raised an issue where he states that even after his manager has been updated, his submitted timesheets still goes to his older manager.
Unfortunately it not quite that simple ; the product is quite flexible in the way that timesheet approvals are handled, so it really depends upon how this is set up in your instance as where to start to look for anomalies.
(for example ; approvals can be done by "anyone" who has the correct access right (if you are using access-right approvals)... OR if you are using an action-item approval method, then you need to look at the process which generates those action-items)
(and I'm talking mostly about the "Classic UI" here)
I believe the first thing you must do is check if there's an active "Project Manager Approval" process running. If there is, check the behavior of that process to see how the timesheets are approved.
By default, this process is not active, but set to have the resource manager approve the timesheets, but in many implementations this process is updated / new version made to change that behavior.