If your signal was at 500 MHz, and you successfully captured it with a 6 Msps sample rate, the signal is no longer "at" 500 MHz. Indeed it was transmitted there, but either:
- your receiver has an analog mixer, and mixed this signal down to baseband such that it could be sampled at 6 Msps, or
- the digital signal was originally sampled at a much higher rate, but has since been mixed to a lower frequency and decimated to a lower sample rate.
A 6 Msps complex-valued stream can represent frequencies from -3 MHz to +3 MHz. At this sample rate it is impossible to represent 500 MHz due to the Nyquist-Shannon sampling theorem.
As such, I suspect you will find that what was 500 MHz (the center frequency of your receiver at time of capture) is represented in the data as 0 Hz.
If you want to shift this around your certainly can, simply multiply it by a complex sinusoid: this will have the effect of "rotating" the frequency spectrum. Unlike multiplying a real-valued signal with a real-valued sinusoid (as done in many analog mixers) this does not generate two sidebands, so no additional filtering is required.