How to Susbtitute all instance(s) of the selected character in any cell using Formulas/VBA

Dear Forum,

I have used the Substitute function quite frequently, but I dont know to get it to replace each character with something else..

Example:

A1= "ABCDEAFGHAAOPLMN"

Now if I need to replace the Letter "A" but all instances of the letter "A" with "*"

How do I do that, all the "A" s get replaces with "*"


Warm REgards
e4excel