You may need to change an ICD10 code if the one you used was not specific enough. Please use our ICD10 lookup to find good ICD10 codes
Once the account has been billed it is not good enough to just change the ICD10 codes on the Account details screen. After doing that you will still need to go to each individual invoice line and chnage the code these too.
If the month has been closed then you will need to Allow changes to a closed month