Step #1 - In the game pass app, go to settings and click manage
Step #2 - It'll open up a browser tab, log in and click manage
Step #3 - Click turn off recurring billing, then turn it off
Step #4 - Go back to the app and go to settings and now click on the Xbox Game Pass link
Step #5 - Scroll down and click on the Extend button, this will let you buy 1 month extension at $4.99. If you haven't turned off recurring billing the Extend button will not be there.
Step #6 - Purchase the 1 month under extend.
Note that this turns recurring billing back on each time. So now you have to loop back and do step #1 again to turn off recurring billing before going and extending another month. Takes about 1-2 mins each month you extend.