Ah yes,
All of the M2TW files are in "Read-only" mode. You must Right click on the original file and go to "Properties" in the Properties menu, there is a check box that says "Read only." Change that to make it unchecked. Now the File is no longer read only and you can save over it with the Original file name.
Let me know if this works.


Now that you have done this, go to your descr_mercenaries1.txt (the one that you created) and save it with the original name "descr_mercenaries.txt" Now it will give the option to overwrite or save over the file. Click "yes."

And your changes should appear in the game.
Good luck.