SPUG: replacing a string occurence in file with other string

[Sachin Chaturvedi]

hi , i am having a text file , say output.txt. in that i am having variour occurences of strings with same extn like file1.data , 
file2.data etc. i want to replace all occurences of files names with .data extn with $testname/file1.data , $testname/file2.data 
etc .