Needing help with electrical plug

Your garage receptacle is not a 50 amp receptacle, nor can it be simply rewired to a standard 50 amp receptacle. If you want a standard 50 amp receptacle, it will need 6/3 wire, as mentioned. You may also want to look in your load center and see if the garage receptacle is interrupted by a 50 amp breaker, my guess is that it is not. There are a lot of these older 240V receptacles in garages. They were used to power welders or other 240V appliances.