Just navigate to the AutoBuy Tab inside your Asset Account.
1. Select Asset Accounts from the Main Menu
2. Click View Details
3. Click AutoBuy (from drop-down on Mobile)
You can change all aspects of your ammo auto-buy there. Be sure to SAVE YOUR CHANGES using the button at the bottom before you exit. Changes WILL NOT take effect until your next billing cycle.
Changes you can make include, adding/removing calibers, allocation percentages for each caliber, and changes to budget, date, and frequency.