Ford Mustangs don't have a dedicated "Overdrive" button in the same way some older cars did. The overdrive function (which is essentially the highest gear in the automatic transmission) is controlled through the gear selector. In most cases, the gear selector simply needs to be in "D" (Drive) for the overdrive to be engaged. There's no separate button to turn it on or off.
If you're having trouble with what you perceive as your overdrive not engaging, there might be a problem with the transmission itself, or there might be a different control (like a winter mode button, or a manual shift mode) that's interfering.
