Remove the cover of the lock case. Using your thumb to hold the locking lever against the bolt, insert the key from below, and turn it, as shown. The bit should push the lever out of the way and retract the bolt. If it hangs up on the bolt, file a shallow notch in the bit's bottom edge so that it turns freely in both directions.
More in Interior
Bench grinder (optional)
1. Penetrating lubricant, such as WD-40