You can edit your reward tier at any time while logged into your Patreon account.
Step 1 - From the drop-down menu on the top right click your pledges and you'll see a list of all your active pledges.
Step 2 - Click edit next to the pledge you'd like to change or delete.
Step 3 - From here you can adjust your pledge amount or reward:
Step 4 - When you've made the desired changes, click update pledge. Your new pledge amount will immediately take effect for any future content the creator shares with you.