I have a few combination door locks from a old demolition job of a school here in the U.K that i worked on. I am now wanting to put one of the locks on a door but i want to change the combination to my own number. When i have looked in the back of the lock i can see the old 4 numbers and one letter that was the old combination because they have been tipped with red paint on back of button. Instead of trying to find combination of old number sequence i would like to use my own numbers.
So if anyone could give me any info how to change combination that would be great

The lock has numbers from zero to nine and letters x,y,z, and c for cancel
Here is a pic of the same sort of lock

I also have a mechanical combination lock but i think the combination lock in picture above look better for a inside home door.
Thanks