Q: What can i do if i forgot my apple id when i want ti activate my ipad after i upgrade ios 6 to ios 9

Are you still logged into the account anywhere so that you can look it up ? Or do you have any of the account's downloads in your computer's iTunes library so that you can do 'get-info' on them and see the id ? If not have you tried : If you forgot your Apple ID - Apple Support ?

If you still can't find it, and if you were the original purchaser of the iPad (i.e. it wasn’t bought used) then you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it