In the terminal, open the file you want to edit with command:
:AE ST.SUBROUTINES C70.S.MY.SUBROUTINE
You need to determine the location of the process you're trying to edit. If it's a FA subroutine, then then instead of ST.SUBROUTINES, use FA.SUBROUTINES.
You can search for the text using "/Text to search". After you find the line, to can go to that line by just typing in the line number. You can then replace the text with "C/"Text to be replaced"/"Text to replace with". After you are done editing, use the command "FIB" to File and Batch, which also compiles the process. Boom, you're done!
You can find more about AE editor here Unidata Command Refs