So SEGA put out a update for Sonic Runners which you have to download if you want to be able to play the game, which is okay as long as you can download it. Except it’s not officially released yet outside of Japan and Canada, so Android users outside of those countries can’t update the game the normal way.
Thankfully, Neowl from our forums has uploaded the .apk file, so just install it on your Android device and you should be able to play the game again. And your save data won’t be lost by downloading it. Alternatively, you can also download the update through the Japanese QooApp store. You should also be able to download any future updates through that as well. iOS users should be able to normally update the game through the App Store.
As for what the update actually does, we don’t know the official patch notes but our forum members have noticed framerate improvements and that the game no longer closes when their phone goes into sleep mode. So you no longer have to worry about your phone going on stand by because of how long the loading screens are.