Hello.
What type of 1.5, stable or dev?
Here you find more information:
If you search in the same topic for “FP3” I think you’ll find something interesting.
Edit: and now also here: FP3: Unable to update from 1.5q-dev to r-stable - "Failed to find update binary"
Edit: I don’t have a FP3, so mine is only a fast forum search and something I have read.