Replaced m air flow sensor now it throws a code that its too lean?

it is a 96 mustang 3.8 six cyl. manual transmission. it started with the code for the m air flow sensor so i replaced that and now it says to lean bank 1 and bank 2. they told me to drive it for a 100 miles or so to let the computer adjust it, i don;t know? i need some advise. thanks ahead of time.