Making a encrypter

Okay so what I am trying to do is create something similar to "Morris Code" They layout of the form is a text box and a button"This button will be called encrypt. Then below that I would make another text box and create another button"This button would be called decrypt. What the program needs to do is make it so that if I type the following text in the encrypt text box "Hi, How are you today?" It could make it so that when I click encrypt it makes every letter move to the next letter in the alphabet and then replaces the "Hi, How are you today" text with the encrypted version that you can copy.

What the encrypt will do:

Say I entered the letter "A" in the text box. When I click encrypt I want it to make that letter switch to a "B" since thats the letter after "A" in the alphabet.

Decrypt would just be the opposite. When someone puts encrypted text in the text box, when you click decrypt it will fix all the letters back to their original place in the alphabet.