If you need to find out your UDID then visit here: https://wallmobile.wordpress.com/2015/02/13/how-to-find-out-your-iphone-or-ipad-udid-whats-my-iphones-udid/
If you already know your UDID then go to the Devices section of your iOS Provisioning Portal and click “Add Devices”:
Enter in your Device Name and UDID (it’s best to copy and paste) and hit Submit
Once you successfully add your device’s UDID, edit the provisioning profile to which you want to add the device.
Select the device from the list
Click on generate.
Download the new profile.