Trying to understand range and frequency of an RFID reader
You cannot change the frequency because it has been tuned for specific frequency, not to mention the legalč stuff even if you could. For given frequency (or better said wave length) there is a relation which determine how far the so called 'near field' extends. And only within this near field passive RFID system could possibly work, because it works similar to the transformer /the energy 'taken' from the primary side indicates the presence of the tag in the field/. The antenna near field extends to 'wavelength / (2 * Pi)'

So if you made the frequency higher, you would actually shortened this reactive detection range. Long range systems (10 m or so) works based on the back scattering principle, much like the radar.