Hi Sarah,
That's not a "basic question" (at least not in the sense of the word you mean!).
The reason that you haven't been able to figure it out is that you can't actually do this in the XNAT OHIF viewer as it stands. The 2-D viewing mode, as opposed to the 3-D MPR mode (split view with all planes) does not currently support switching orientation. However, this is likely to change in the not too distant future, when a forthcoming release will utilise the capabilities of
OHIF 3.
Sorry we can't give any timelines at present, but watch this space.
Best wishes,
Simon