How do I fix a tilted horizon?

Fix it in the photo, not in Panoramr: the tour editor has no tool to level a horizon. Straighten the picture in the app or program that made or stitched it, export a new flat panorama file (twice as wide as it is tall), and put it in with Replace Original File.

To do that, open the tour, click the Panoramas tab and select the scene in Panorama List. Click Replace Original File, stay on Upload New, tick Regenerate Thumbnail? so the thumbnail shows the straightened picture, and drop the new file. The scene keeps its name, hotspots and links.

Hotspots are stored as angles on the picture, so after the horizon moves a hotspot can sit a little off. Check each scene you replaced on the Hotspots tab and drag any that moved. Fix a tilted horizon has the full steps.