If we allow direct changes to the plan line/s then the whole link to a usage and a price will be broken and we will not be able to prepopulate future plans properly.
The recommendation is to change the unit price if possible. The prices shown initially are estimated using historical information and are the most likely source of errors.
If you know the total cost for the month then divide by the expected usage shown to find the unit price that will give the value, you want. The diagram below should help with how the formula shown at the top of the page can be used to work out other values.