For an electronic device the the MTBF requirement is 4500 hours per year for 10 years, what does this MTBF mean and how to convert it into hours?
The statement is rather ambiguous. I think it says what the MTBF is supposed to be after 10 years if you operate the device for 4500 hours per year. If so, that's a long-about way of saying the MTBF is supposed to be 45,000 hours. Perhaps this is supposed to give some indication of both elapsed time and running time. Some devices wear out due to time alone, but at a different rate when operated, so there is some sense to this.
MTBF means mean time between failures. If you get a large number of devices, then on average that is how long each runs without a failure. Conversely, if you get a large number of devices and run them for the MTBF period, half should have failed by the end of the test.