OK. This is a very basic routine. I'll include it here, since many people doesn't know about its existence.
As a little add-on, this routine supports also lists of search/replace terms.

For more powerful operations (eg, regular expressions, lots of MB of text to search-in, etc.), take a look to the Satimage osax or any shell utility (grep, etc.).

OS version: Any

Applescript:

(*
searchReplaceText(searchTerm, replaceTerm, theText)
Replaces a string found in a text by another string.
This handle supports lists of search/replace terms.

Parameters:
searchTerm: the text to search for
replaceTerm: the text to use as replacement
theText: the text to search/replace

Examples:
searchReplaceText("foo", "bar", "You are a foo") --> "You are a bar"
searchReplaceText({"foo", " a "}, {"bar", " one "}, "You are a foo") --> "You are one bar"
*)

to searchReplaceText(searchTerm, replaceTerm, theText)
   set searchTerm to searchTerm as list
   set replaceTerm to replaceTerm as list
   set theText to theText as text
   
   set oldTID to AppleScript's text item delimiters
   repeat with i from 1 to count searchTerm
       set AppleScript's text item delimiters to searchTerm's item i
       set theText to theText's text items
       set AppleScript's text item delimiters to replaceTerm's item i
       set theText to theText as text
   end repeat
   set AppleScript's text item delimiters to oldTID
   
   return theText
end searchReplaceText