There isn't really a way to unlock it if the combination was forgotten. You should always right down the combination the first thing you do, and hide it somewhere.
Trial and error. Start at 00001 and work your way around. You could probably do it while watching television over a period of time, do a bit every night. ****** That would work, but it would take a...
Often there's a pin-hole you push in with a pen or a needle and release it at the location of the new combination. You may have to know the old code in order to be allowed to set a new one.
By definition, a combination lock cannot be picked, as it has no key. It can be "cracked" in some cases where putting pressure on the lock and diagnosing the dial can reveal the combination. It can...