https://itch.io/docs/legal/terms#9-refunds
If you are really unhappy with a purchase, you might wanna try anyways, but this is not a try-before-buy feature, as some people use it for on steam.
Many projects here are pay what you want.
And what they do not tell you, you cannot increase the payment value after you bought something. It is a bug/design flaw. You will just by it a second time. Some pages have content that says, you get it if you pay 10 $ for example. So if you donated 5 $ before, you cannot just donate 5 $ again.
So best try the free stuff on a page, and then decide how much you want to pay or overpay or tip.
Of course some projects are plain old paid projects and have no demo version. So maybe look twice, before going through the hassle of a refund.