how do i change the order of photos in the browser in final cut pro?

Do you mean in FCP browser or in the Photos browser? In FCP you can set that using the menu at the top

If you mean inside the albums in the Photo browser, that's probably set in Photos. You could also bring all the photos you want to work with into FCP and use it's ordering functions.

I can't see a way to change that...

A possible workaround - not ideal: add the photos to the timeline, adjust timing if needed, make a compound clip (Option-G) and then use the Retime tool to reverse the compound.

I gather that. I would suggest to drag them into the FCP browser, which should be the standard procedure anyway, and sort them there before putting them in the timeline, rather than dragging them directly to the timeline.

IIUC, the OP is referring to the Photos browser inside FCP, where we don't get these controls:

Another workaround: open the same Album in the Photos application, where you CAN change the order; select all, and drag to FCP timeline. This way the order is maintained.