WAZA-AIR: Can an accidentally overwritten preset patch be restored?

Yes.
You can use the following procedure to restore it.

1. Start “BOSS TONE STUDIO for WAZA-AIR” app.
2. Tap the “TONE CENTRAL.”
3. Tap “WAZA-AIR Preset Patches.”
4. Select the patch that you want to restore, and tap the [ADD] button.
5. When the "Completed!!" indication appears, tap the [OK] button.
6. Tap the [LIBRARIAN] button.
7. In the upper part of the screen, tap the [EXPORT] button.
   
8. Tap “WAZA-AIR,” and then tap the [NEXT] button.
9. Select the live set that you want to export.
10. Select the patch that you want to restore, and then tap the [OK] button.
    “ALL PATCH” exports all patches of the liveset to the WAZA-AIR unit.
    “SELECT PATCH (FROM TO)exports the patches between “FROM” and “TO” to the WAZA-AIR unit.
    “SELECT PATCH” exports the selected patch to the WAZA-AIR unit.
11. Select the patch within the WAZA-AIR at which you want to start overwriting, and then tap the [EXPORT] button.

For details on operation for steps 6 and following, refer to "[WAZA-AIR Reference Manual] Using BOSS TONE STUDIO (iOS/Android)," in the section "Exporting a Live Set from Librarian to the Unit (EXPORT TO WAZA-AIR)."