You can try the schnapps-based upgrade mentioned in Migrating to TOS 4 with schnapps . However, please, keep in mind that doing any kind of upgrade to Turris OS 4/5 will not migrate your configuration (yet) and you might lose all data on the router’s internal memory. Turris OS 4.x is production-ready, while Turris OS 5.x is in a kind of beta stage. It should be practically usable, though, for the majority of people.