Hi there. I donated for one teir a few days ago and decided to donate for the next one. How should I go about it to make sure I get credit for the higher teir?

So sadly, you can't add up your two donations to get a higher tier meaning that if you've already paid 10 dollars you can pay another 40 dollars to get to the 50 dollars tiers. Instead, you'll have to pay 50 dollars to get the new tier. 

I wish you could do that but sadly Itch does not allow that!