replace a line in a text file with sed

I have a text file which defines the values of some important variables. Such a text file is used for several simulations. I need to change the value of one parameter in the file:
ap = 24.1, to a smaller value 3.

what I do is to use the sed.
sed 's/to_be_replace/replaced_result/' text file

mv text_file tt
sed 's/ap = 24.1,/ap = 3,/p' tt > text_file

then the old text file is modified with the ap value !