How can i customize the severity values for particular product?

I have used all the severity values for a diferent product while assigning the bugs in a different project.
Now i want to make it simple by having only these values{Blocker,Critical,Major,Minor} .i tried deleting the others,but its not possible as there are bugs assigned to these.

I want to know how i can set these values only to the new project.?Can i useclassification?what is the purpose of this?Or is there any way i can acheive this?