How to replace a group of symbols to another symbol?

Questions
,
1

Hi,
I’m beginner in Mathematica and apologize in advance if my question is foolish.

I have an expression:
1 + f/F - (f S)/F^2

and need to replace f/F->m, and to have in the end
1 + m - m*S/F

What should I do?

2 
(1 + f/F - (f S)/F^2) /. f -> F m