String Replace in C#

Question

How to replace the first "AA" with "XQ" in the following strings

"AA1"
"AAAA1"
"AAQQ1"
"AAAAX1"

So the out put will be like

"XQ1"
"XQAA1" 
"XQQQ1"
"XQAAX1"

i don not know how to do this. Can any one help me to do this?