Issue with the sales tax in nav 2009r2..

Dear Friends,

I have an issue with tax c form in purchase order. That is i have created a purchase orders with c form ( tax 2%). Then i have received the quantity. But at the invoice time i got the information from vendor.

Tax is 5% not 2%. But i was unable to change the tax %.

how can i change the 5% to invoice the purchase order.

Can anyone please help me …