I agree with the above answers and if these answers are not working then I guess your accounting document has already been created for the billing document.
If yes then you can not change the price in billing document because after generating the accounting document, system will allow you to change only one thing in billing document that is "Output".
If you still want to change the price then go to t-code VF11 --> enter your billing document --> Enter --> save it.
Your billing document will cancel and then create a new one in VF01 and then you can change the price.
Hope it helps,
Yes you can change Price at Billing Level.. Create new Condition Records(Vk11).before you create Billing Documents.and
Once you create Billing Documents(VF01)--
> one you Execute
->click on Header details and go to ....>condition Tab page-- > below you can see ---> you select and click on carry new Pricing..(now the new price will effect)
>update Puch Button
Same like this we can also do for Items.