If you expand on a Main Category that has a budget created within it, you will see a "Manage Budget & View Transactions" link for each budget item under it – it is here where you make edits or changes to your budget.
When managing your budget, it is important to note that the budget will recur monthly and indefinitely by default. That is, unless you check the "For this month only" check-box.
You can also edit your budget for just one month if you want and it will not affect any future months. If you originally created a default recurring monthly budget, you can edit that budget for just the current month by clicking on the check-box and it will not affect the budget for the future months with regard to how you originally set it up.
Lastly, if you no longer wish to have a specific category budgeted simply click on the "Delete this Budget" link in the window and it will no longer recur monthly.