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 (for patrons this is your profile icon and for creators, this is the three line icon) 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 head to "Your Creators" using the bottom navigation.
Once there, swipe left until you get the creator that you wish to update your pledge for. You'll then see three dots at the top-right - click them! You'll see a pop up that denotes "Edit pledge." You'll be taken to the page to delete or edit your monthly pledge.