if it is not available decend to warmer air.
Also in many cases climbing too a higher altitude. Doesn't seem to make sense but a lot of times air can be warmer higher up.
This was a question I posed to a flight instructor, "What should you do if you notice icing and your pitot heat isn't working? " He said "Go too a lower altitude", fair enough but "What happens if you suddenly find a bank of clouds as far as you can see, below you, you have minimal instrument training (or they aren't working), more or less flying blind?"
He said he wasn't sure. So I got to doing some research (most instructors don't like me because of some of the senerios I come up with

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From my research I found that by climbing higher, in more cases than none the air will be warmer than at the "icing" altitude!
So I posed this info to a FAA Examiner and he verified that yes it would be true.