Although you can't change the category of a budget you already created, we've got you covered with a workaround that will also do the trick!
To get the result you're looking for, change the categories of the transactions themselves and then add a new budget for the category you chose.
So, if you want to change your fast food budget to restaurants, change all the transactions that are categorized as fast food to restaurants and then add a budget for restaurants (if you don't already have one). You can also create a rule so any fast food transaction will always be categorized as restaurant.
For more information about editing multiple transactions, click here.
For information about creating rules for transactions, click here.