To change the PPP calculation navigate to Configuration > Positioning. Within the Positioning menu page changing PPP Mode will change the type of PPP solution computed by the LD900.
PPP Mode can be set to Apex (default) or Ultra by clicking on Edit and using the front panel arrow keys to select Apex or Ultra as required, followed by tick:
Then use the arrow keys and tick to select Apply, right to select Yes and tick to apply the change. Note, changing the PPP Mode will re-initialise the PPP calculation, which will take time to settle before a PPP solution output becomes available.