It's been said that Docomo phone are easier to update, only Japan VPN needed. But in reality it's not that simple. Even after using VPN, most of the time you wont be able to find OTA update through system update. Sure factory reset can help you to get update, but keep resetting phone for each update is unproductive and bothersome.
And nobody bother to actually write down the detail instruction on how to update Docomo phone (in this case is Aquos R6)
So here you go, the needed steps to update without factory reset your phone.
1. Download Proton VPN, it's Japan server is fast and free
2. Take out SIM card from the phone (it will automatically restarted by doing so)
3. Connect to fast WiFi
4. Open Proton VPN and connect to Japan server
5. Turn off WiFi but don't kill/close your VPN app
6. Go to Setting, type in search field : "Google Services Framework" , tap it, choose Storage, and do Clear All Data & Clear Cache
7. Now search again, for "Google Play Services" , and choose storage again, tap on Manage Space and choose "Clear All Data"
8. Go to System Setting again and change your region to Japan,
9. Now connect back to WiFi
10. Check Proton VPN status on notification, make sure it's back connected to Japan Server
11. Now go to System Update in setting and tap "Check for update"
12. That's it, you will be able to get the OTA update and installing it now.