File naming function in c#

I would like to make a function that takes two strings passed to it and renames it and passes that new name back.
the input strings it will get will be

parameter 1 in "region 1" or "region 2" or "region 3"
parameter 2 in "originalfilename" an ex: "UNArpC1C6A03"

in the filename the c1c6 is constant but text to left and right of that will change

if
 "region 1" then output becomes "UNArpC1C2A03"
 "region 2" then output becomes "UNArpC3C4A03"
 "region 3" then output becomes "UNArpC5C6A03"

end


so another example I pass
"Region 1" and original filename "UNLeadC1C6A101"

will output "UNLeadC1C2A101

"region 2", "UNLeadC1C6A101"
will output "UNLeadC3C4A101"

"region 3", "UNLeadC1C6A101"

will output "UNLeadC5C6A101"

I dont know C# so dont know how to start this