Macro code for automatic change of a cell value

Hello.

I need a macro code to change automatically a cell value.

More precisely, I would like to put a value (text or number) in a cell, and
after leaving it, the value in the cell automatically changes into something else.

Example: I want to have the "morning" text in a cell;
I will put the letter "m" in it and will press Enter.
As a result, I would like the word "morning" to appear in the cell.

Some help please?

Thanks in advance.