You can move budget amounts between your categories while keeping the total budget amount the same for the month. This can be helpful if you overspend in a category but have available funds in a different budget category.
Take this example where I've spent $65 on Shopping that I didn't originally budget for. That's ok, I can move money from another category where I have remaining funds available.
By clicking on the red $65 I can pick another category to move money from. In this case I'll take it from my Personal budget.