Settings → Security → Enable "Unknown Sources" (or "Install from unknown apps")
To download and update Summertime Saga 0.20.17 (often referred to as part of the "Tech Update") for Android, you can find the official APK files through the Official Summertime Saga Download Page or reputable third-party mirrors like Key Features of the 0.20.17 Update
Settings → Security → Enable "Unknown Sources" (or "Install from unknown apps")
To download and update Summertime Saga 0.20.17 (often referred to as part of the "Tech Update") for Android, you can find the official APK files through the Official Summertime Saga Download Page or reputable third-party mirrors like Key Features of the 0.20.17 Update