1. Go to Reaper / Preference
2. Choose Keyboard/Multi- (touch) from the menu that appears.
3. Choose Assign keyboard shortcuts to actions or change existing shortcuts
4. In the window that appears select the command you want from the list or use the filter to find the command you want. Then click Add from the Shortcuts for selected action menu (Image below).
5. Type the short cut key you want into the Shortcut field and click OK.