If your lock is still working good and you just want to change the key, then you can take it out and go to homedepot or home hardwear, they can reset your lock and make two new keys for you, I think this is the cheapest way.
Yes, last time when we need to take out the lock to rekey, we change the one which on the garage door to the main door, then after rekey the main door's lock, we change it back again, it is easy.
