Under Source Images, you just have to hover over the pano you want to replace, and you'll see these icons appear:
Click on the icon in the top right, then browse & select the pano you want to replace that image with.
Repeat for all the panoramas you want to replace.
This will work even if your tour has already been built. To refresh your tour with the new panos, just click Edit Tour, then click the refresh button on the top right. Then you can publish/republish your tour, and everything should work well.
NOTE: If your tour has already been built, it's very important that your replacement pano should be located in exactly the same spot as your original panorama. Otherwise your project dimensions won't be accurate, and you may need to rebuild your project again.