Where can you modify it?
In the Tour Editor, under Model View, you'll see this:
You can change the texture quality by modifying the options in the right pane.
3D Model Texture Resolution
If you click the dropdown, you'll see the following options:
The lower the number, the lower the model's resolution and the faster your tour will load.
Keep this low if you're expecting your viewers to have slow internet connections. Keep it high if internet speed is not a concern, and you want your model to look as nice as possible.
Checking this option will make your model look much nicer, as Stage will try to layer in the covered/hidden spots in your tour with visual information from other panoramas. It'll take much longer for your model to load though.
After you've configured your resolution and quality settings, click this button to load your new model.
The following popup confirmation will appear:
Click OK to proceed, or Cancel if you've changed your mind.