Difference between a frequency synthesizer and a frequency mixer
A frequency synthesizer does exactly what its name implies.  I synthesizes a frequency.  Often these are adjustable so that they can generate a range of frequencies.
A mixer, on the other hand takes two signals and produces an output which amounts to the sum of the two and the difference of the two.
So this would be useful for your upconverter.  If you take your 915 MHz signal and mix it with 1.485 GHz you will get two outputs, one at 2.4 GHz and another at 570 MHz.  If you then high-pass filter the output you will be left with the 2.4 GHz signal.