To encrypt a message with a Caesar cipher, each letter in the message is changed using a simple rule: shift by three. Each letter is replaced by the letter three letters ahead in the alphabet. A becomes D, B becomes E, and so on. For the last letters, we can think of the alphabet as a circle and "wrap around". W becomes Z, X becomes A, Y becomes B, and Z becomes C. To change a message back, each letter is replaced by the one three before it. However it is very easy to break a Caesar Cipher using Microsoft Excel. Using a more difficult or complicated encryption would be better.
Changing by three is the rule that Julius Caesar used, but the same idea works for any number. ROT13 is a modern version and even easier to break.
The Caesar cipher is a substitution cipher: each letter is replaced by another. No substitution cipher is really safe.