You can't change directly because its LNA and mixer ( and apparently PLL )have been optimized for this frequency.But you may use a external mixer-oscillator to downconvert this signal.In this case you should use an LNA that will operate at 900MHz.
You may want to mention the receiver you are using. For some popular SRD devices, changing a few components (L and C)
and software register values does the job.
As BigBoss said, the 433MHz unit isnt going to work on 900MHz
you need to mix the 900MHz signal with a separate LO (local Oscillator) to produce an IF of 433MHz.
( that is if you insist on using the 433MHz unit)
Else you just build a complete 900MHz receiver