How do I change two spaces to one space in my document?

One of the last prep things that most people ask to do is how they should remove two spaces and have only one between sentences.  
While people will sometimes say it doesn’t matter, if you want to, in Word, you use Ctrl+h (find and replace), Pages is easy to find it in a menu, same with Google Docs, and type two spaces in the find box and one space in the replace box.  Run it a couple of times, and then you can be sure you’ve got all of the doublers (because sometimes people type three or more).  Formatters may or may not already have that function as part of their work, so you might find that it’s uneccisary.  It’s also not something to worry about because it’s easy to fix it.