If you: don't sync notes but store them on your device ; and want to backup your notes to restore later on a fresh install of Ubuntu Touch (where you chose to wipe all):

In 16.04, notes are stored here:
.local/share/com.ubuntu.reminders/@local In 20.04, notes are stored here:
.local/share/notes.ubports/@local

You can use File Manager app to create a backup without need for terminal:

Open the app and next to Restricted Access click Unlock and enter password; Go to the location above; Copy the @local folder by sliding it to the left and then click the copy icon; Paste it somewhere in your Documents folder (from the menu, top corner) Attach your phone to your PC and copy the backup folder to your PC.

Once you've got the got a fresh install with Notes app:
Follow the above steps in reverse and replace the newly created @local folder with your backup folder.

(I know the OP asked this question long ago, but this answer might still help someone with same issue).