how to sign in icloud for ipad activation

Use your iCloud ID and password. Is it possible this is a second hand device and it is asking for an iCloud ID that is not yours, or you do not recognize? What happens when you try to sign in with your iCloud ID? Please explain more about what is going on.

I can't remember my icloud details so cannot activate ipad after updating it

Then you can start out at https://iforgot.apple.com and recover your information. Without it, you will not be able to activate the device. If you are the original owner and cannot recover, you can make an appointment at the Apple Store and bring the original receipt and identification and request assistance from Apple to unlock the device.