Are you asking where to do that, or are you having trouble doing so with your account? 

You may want to refer to the manage your account FAQ. There have also been cases where people are trying to sign in using the forum account, rather than the Sonos system account (different emails). You may also want to check your Sonos system for updates. 

If all else fails, you should call Sonos Support directly to discuss it.

I only use one e-mail and I am signed in, but the app is asking me to sing in as a system owner.

It is an endless loop.

So, what did Sonos Support say, when you called in, as suggested?

Sonos doesn't have a call center in my region. 

I was gonna get a full set of Sonos speakers, but now I will NEVER buy or recommend another Sonos product. 

Total failure!!!