By the flowing instructions you can easily change multiple letters or texts case upper , lower or proper in ms office.there is three way to do this. to Know how to change letter case in excel here

1- Just highlight your texts hit shift +F3 key to change to upper or lower case.
2- Highlight your letters click right button mouse font => Effects => tick mark in small caps or all caps you see sample in preview during tick mark
3-Go to home tab => font see this photo

change case upper lower