Once a budget has been created, this is not set in stone and can be easily edited. To get started, click into the account the budget exists in followed by clicking into the budget itself to show the following screen.

From here there are a number of options for editing individual budgets.

The date field can be changed, either within the same month or to a totally different month

The amount can be adjusted

The eye symbol, allows you to exclude that budget occurrence from the cash flow, or re-include it if it has previously been excluded. 

The cross symbol allows you to delete that single budget occurrence

The plus symbol, below the budget tile, allows you to quickly add in a new budget for that month
If you would like to edit multiple budgets using a batch action, find out how here.