Hi there, is the old job closed in WF? Depending on your hour preferences, you may or may not be able to revise time on a closed project. You can try going into that project's Hours tab and see if you're able to remove the incorrect hour. If you submit timesheets and you've already submitted, you should be able to see a 're-open' button at the bottom of the timesheet where you would otherwise see 'submit' or 'close'.
If this helped you, please mark correct to help others : )