Microsoft Word very specific technical question?

So i'm writing a book but I really want to change one character's name. The name is like Alex or something and I want to change it to Garrett but the name appears like 40,000 times and I was wondering if there was an easy way to just change that one word and every time it appeared in the book it would change too. So like if I changed Alex to Garrett once it would change it to Garrett every time you see the word 'Alex' any way to do that? Thanks in advance I appreciate it a ton!