Safest to back up the original files (simply change their names to "button_ipnext.bak" etc) then copy in the new ones. This will save you over-writing the originals. The if it does go pear-shaped, you can simply delete the new copies, and change those two back to "button_ipnext.bif" - it will warn you about changing file extensions, but that's fine, it's what you need to do