2013 Ford Focus knee airbag replacement?

I bought a salvage rebuildable car from copart auto action that was in a minor crash resulting in minimal front end damage and the drivers knee airbag to go off. I put in a airbag that hasn't gone off but now I'm stuck on what to do. Do the impact sensors need to be replaced along with the module? Or does the module just need to be reset or flashed

Never buy a car with blown air bags. See a mechanic. Air bag maintenance/repair is not for amateurs.

That depends on what the manufacturers say. Some recommend replacing the sensors since they may have been damaged, others say the sensors will reset and can be re-used. Personally I'd replace them to be on the safe side. It is NOT recommended to try testing the airbag circuity with a multimeter and probe, since that may cause the airbag to go off.

If you're going to be working on the airbag electrics be very sure that the battery is disconnected. A mechanic I know insists on turning the headlights on after disconnecting the battery to dissipate any charge there may be left in the vehicle systems and going away for 10 minutes before starting work. Whether that's necessary or not I'm not sure but it certainly won't do any harm.

When air bags deploy you need to replace the air bags, the SRS control module and the seat belts.

Yes, the sensors should h ave been replaced.