Find and replace in Linux text file

[rfcn]

Hi,

I'd like to find and replace some strings in a text file in Linux. It's a condition replace. For example, if there is "X=100" in a line, then replace 111 to aaa in the line. See bellow,

111222333,X=100
111222333,Y=100
111222333,Z=100

I want a script, the script will excute the find and replace and the text will convert to:

aaa222333,X=100
111222333,Y=100
111222333,Z=100

How to rearlize the script?
Thanks!