How do you open sentury safe without key?

Firstly, is it yours? If not, then you can't.

Contact the manufacturer and see if you can get a replacement key. If not, call a locksmith. They can usually pick the lock open. If not, they can also drill it open. If they won't drill it, look up instructions on how to drill out a lock. If that's not possible, see if it can be sawed open. If not, hire a company to blast it open. If all else fails, re-evaluate the value of the item inside the safe. Is it really that valuable?