In Word, selecting some text and pressing Shift+F3 will change the case of the selected text. It works on a cycle through the following – lower case , CAPITALS or Sentence Case.
Excel does not have this feature but you can select a cell and use =UPPER, =LOWER or =PROPER.
The format is =FUNCTION (CELL) where FUNCTION is the required function (upper, lower or proper) and CELL is the cell that contains the text.
For example, if A1 contains MIKE THOMAS, and in B1 you enter =PROPER(A1), B1 will contain Mike Thomas.
You can then use Copy and Paste Special / Values to copy the contents of B1 into A1, thus overwriting the original text.
I’ve used this feature many times for clients who have been given a list of, for example, names, that have been pulled off another system (say a payroll system) where the original data has been stored as capital letters.