Open Billing Account in Billing Care and then click 'Bills'

Scroll down and click the service charge you want to adjust

After selecting, click the button beside 'Show Charge Changes' and then click New Adjustment

Adjustment modal will be shown, to adjust, do the steps below:

1.) Exclude taxes from the adjustment should always be unticked.
2.) Input Adjustment Amount
3.) Select Effective Date
4.) Select Adjustment Reason

after finishing these steps click 'OK' button