I tried to change it to $0 and it just reverts back to $1. Some players (especially those who don’t know English very well) thought the game isn’t free and paid because of it and I want to stop that by setting default donation amount to $0 please.

I don't know how to fix your problem but I do now how to avoid it in the future. when you go to create a new game like when you make a title and upload the files and all. if you scroll down a bit you should see pricing. you can set it to No payments. 

Sorry, I meant the default donation amount because I do want to accept donations of course, I just don’t want people to mistake the donation as the required money to pay to download the game.