If you are already a donor, you can just upgrade your donor tier here.
If you're a Tier 1 Donator, you can upgrade your donor rank to Tier 2 for just $15 more.
If you're a Tier 2 Donator, you can upgrade your donor rank to Tier 3 for just $15 more.
If you're a Tier 3 Donator, you can upgrade your donor rank to Tier 4 for just $15 more.