To increase the amount in an existing approved budget, all you have to do is Create a New Budget and while uploading the excel, choose the same values for the budget attributes and provide the amount under the Budget Cost to get this amount added to the existing budget amount.
Specify the (-) symbol if you would like to reduce the existing budgeted amount as shown below.
For such a reduction in the budget amount, a negative amount will be displayed in the Budget Amount column.