WordPress tells me that the fourth most common search string that leads people to this site is “changing ubiquity starting keystrokes”. Huh. That’s interesting. The fact that it’s such a common question probably means that the feature is too hard to discover. But as long as people are asking it, maybe this site should, you know, answer it or something.
Do the “Help” command in Ubiquity. A page will open; in the upper-left corner of this page there is a widget for setting they keystroke combination that activates Ubiquity.
If you can’t bring Ubiquity up, because its default keystroke combination doesn’t work on your computer for whatever reason, then you can get to the same page by typing about:ubiquity into your URL bar.
Hope that helps.