custom button - opportunity

I have created a custom field called discount on the Opp Prod revenue. I would like to take off Opportunity Update totals button, create a new button which will deduct the discount from the purchase price*qty and update the revenue at the header level.
I  came across some of the postings that suggest to use Ajax/Jave code to accomplish this. Any help with the code would be of great help to me.

Thanks,
SKJ