1/ Install SyncThing 0.13.9 using available package
2/ Upgrade to version 1.0.1 manually with below command
wget https://github.com/syncthing/syncthing/releases/download/v1.0.1/syncthing-linux-arm-v1.0.1.tar.gz && tar xzf syncthing-linux-arm-v1.0.1.tar.gz && rcsyncthing stop && cp -a syncthing-linux-arm-v1.0.1/* /opt/syncthing/ && chown -R syncthing:sync /opt/syncthing && rm -rf syncthing-linux-arm-v1.0.1* && rm -Rf /var/lib/syncthing/ && rcsyncthing start
3/ tail -f /var/log/syncthing/syncthing.log to see upgrade process
4/ delete the db folder that can't be upgrade : rm -Rf /var/lib/syncthing
5/ Restart service syncthing in ALTF GUI if stuck in log or service does not start
6/ Upgrade to version 1.30.0 using GUI upgrade button
7/ tail -f /var/log/syncthing/syncthing.log
8/ Restart service syncthing in ALTF GUI if stuck or service does not start
I was unable to upgrade directly to v2.0.1 because otherwise can service will never start even if you delete db.
I think there are intermediate versions that you should upgrade to in other to reach last version.
I will try to find a way to upgrade to latest version 2.0.1 later
Hope it helps.