If you made a mistake when validating your timesheet, don't panic!
It is up to the client to refuse your timesheet so that it can be reopened. You just have to contact your client so that he does not validate the timesheet.
Please note that if the last person on the Client validation workflow has already validated the timesheet, you will not be able to modify it.
At this stage, for any modification, you will have to make a regularization on the next timesheet.