Last tested: Sep 14, 2020
Github's UI makes this pretty easy, and they have an article on it.
Basically, you want to go to the repo, click on the file in question, and then select "Blame" or "History."
"Blame" will show you the changes made on a line-by-line basis, whereas "History" will show you a timeline of changes. Think of it as sorting changes by "line number" vs sorting by "change date."