i did update to my iPhone 5s after update it is displaying start page asking for apple id and password i have forgot the id but i know the password how to know my Apple id ??

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 your id, and if you were the original purchaser of the phone (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 phone 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

You can attempt to recover the ID at https://iforgot.apple.com