How you can enter equ?

Depends on the type of the actual constants. If this number is, it will fall to the registers and had to change would be opcode. If this data is compiler dependent. Some can pack the constants in the data section and they can be changed. But is who creates the partition with read-only access.

Constants defined using the equ is character constants, their values are substituted at the stage of preprocessing, you can define them and redefine anywhere, their place of action, from the point of definition to the end of the program.