You can edit your pledge 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, reward, etc.
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.
For the mobile app, log in and go to the account page. Once there, click on "your creators" and then click on the creator that you wish to update your pledge for. You'll then see a link at the bottom that states "You're pledging "X" per month." click that and you'll be taken to the page to delete or edit your monthly pledge.