DO you mean you are paying a monthly direct debit and want to alter it? If so this cannot be done on ROS. You must complete a new DD mandate and send to Revenue.
Your can email the DD section at cgdd@revenue.ie with your requrest to change your monthly DD amount.
Just include your RSI number, Name/Company Name, Bank account details, Current DD amount, Revised DD Amount and the date from which you want to change the DD amount.
I have used this email address for a number of years and changes have always been put through immediately.
The Phone number for this Revenue section is 065 6841200