First of all, thanks for your effort to spend your precious time to upgrade to out latest version which surely will bring the best experience with great features in order to make your life easier.
Normally Maintenance Notice page will be displayed only during our down time when we upgrade our servers and database to latest version, and back-end app should come back to normal status.
However, in very rare situation, caching issue happens, and you can use Ctrl + Shift + R to resolve it simply.
In case Ctrl + Shift + R does not help, we highly recommend you to try with Basic mode in Chrome to clear browsing data (by clicking simultaneously Ctrl + Shift + Delete), and choose Clear data button in the next step as the following:
Hope it can be helpful to you.
If you need some more helps, please feel free to contact us via live chat or email us at firstname.lastname@example.org