Batch replace

I have problem with batch replacing a text file

example: a file name :"first .txt" has the following text

A 1
B 2
C 3

another file "second .txt" has the following text
"r if this is your first time here, and if you wish to include example C++ code in your posts and messages, please surround your example codes"

task is where ever "a" is there it should repalce with 1 and "b" is there should replace with 2

can any body help in this