Hello Guys!
Here are steps to do that on your Android Phone
1 - Open your chrome browser, click on the web address & type in Youtube.com (make sure you're logged in to your YouTube gmail account)
2 - Click on the three dots at the top right of your browser, scroll to desktop site & tick the box
3 - Click on web address again and click on the edit Icon, now type in features & click Enter
4 - Now look your Screen closely, you will see "FEATURES THAT REQUIRE PHONE VERIFICATION", next Click on it and click on Verify phone number
5 - Select your Country and add your phone number then click "GET CODE" (finish the verification process)
6 - Now download YouTube studio and open the app
7 - Select the video you want to add thumbnails to & click on the pencil icon (edit), click on the pencil icon at the top left of your screen & Select the thumbnail you designed & click save.
Watch the Tutorial below to understand better, Thanks for your time ❤❤

https://www.youtube.com/watch?v=pidh-2QTdlI
YOUTUBE CHANNEL NAME - REXYNE
https://m.youtube.com/channel/UCXQQOb-z3rqJSC1ig6nL5Uw/videos