When the combination of  safe is not available the only option left is to crack.  This can be done by two different methods.  One is by touch and feel ( the old fashion way).  This will work on most older locks and some low grade new locks but it is a skill that take a long time to develop.

The other method is via penetration of drilling   Now if you are thinking that this will be easy you are wrong.  This is not only difficulty but dangerous.  Bits can bind and break, you can damage the lock and set off relockers.  Relockers are special spring loaded bolts in a safe that are designed to fire into the side if the lock is damaged.  If you set off the relockers you have just quadrupled the time it will take to open the safe if you can open it at all.

There is a good school on safe cracking and maintenance and it is recommended that you go to it or get trained as an apprentice before working one most safes.  MBA USA is the company that offers the course and it is reasonably priced.  Their courses are taught in their office and are hands on.  It is a good idea to get their catalog.  They carry a lot of high end locksmith  tools.   High end equals high quality and high cost.