Press spacebar twice at the end of each word. Or, after finishing the
document, replace one space with two. But why in the world do you want to do
this?
If you want to do this occasionally, a search (for single space) and
replace should do it. Or assign a macro inserting double spaces to a key
board combination and use it when you are finished typing a word. But I
would be interested to know the reason why you want to do that?