2.5 height map and noise value ?

where do you find this noise value in 2.5 ?

now is there a way to add or subtract this noise value?

i mean right now seems that you can only add noise!

and it it is too much then you have to erase or redo the file ?

might be easier if we could add and or subtract this noise ?

or may be there is another way to do that but how ?

anythng in 2.57 that wold make it easier or more flexible to use heightmap?

thanks happy 2.5