If you don't know how to open a lock cylinder in your hand, you probably should take it to a professional locksmith. They don't charge much to rekey a lock that you bring into the shop, and they can usually do it in under ten minutes. Twelve minutes if you don't bring a key that can open it.
That said, you can open it by shimming with a key blank and a shim, bumping with a bump key, removing the slide or top of the cylinder, picking with a pick, pick gun, or electric pick, key picking it, using tryout keys, rapping it open with a hammer handle, or simply wiggling every old key you can find in it until it either turns or you run out of old keys.
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If you don't know how to open a lock cylinder in your hand, you probably should take it to a professional locksmith. They don't charge much to rekey a lock that you bring into the shop, and they can usually do it in under ten minutes. Twelve minutes if you don't bring a key that can open it.
That said, you can open it by shimming with a key blank and a shim, bumping with a bump key, removing the slide or top of the cylinder, picking with a pick, pick gun, or electric pick, key picking it, using tryout keys, rapping it open with a hammer handle, or simply wiggling every old key you can find in it until it either turns or you run out of old keys.