You can’t, you might be lucky using one of the desktop apps.

Yes, I can’t find that option either.  However, the Sonos app on my PC allows me to enter the library path under the Manage tab and the first option on the pulldown menu Music Library Settings then click the Add button on the right. Then select what device your music is on, for example, mine is on a WD Passport NAS drive, so I select the bottom option for Networked Device. I entered the complete path, since it is pointing to where my iTunes directory is pointing to but the only problem is that the next Sonos screen brings up a username and password. I don’t have a username and password on the Passport drive so right now I am going to contact Sonos again and tell them this. This might be a bug in their new app. 

This is my issue as well.  The music library appears randomly on the iPhone app, but always appears on the web app.  However, I can’t find where the pathway is entered or stored.  I assume it just migrated over from the old app, but if I want to change it, I don’t see any way to do so.  I don’t see a Manage tab on the web app as it appears on my screen.  This is just one of the many infuriating aspects of this new “enhanced” app.

Same issue here.  My system is on the correct wifi network, but the new iOS app does not show or provide access to my music library, so I now cannot play any of the music stored on my NAS. 

@Sonos Team:   make this a priority to fix this please !!   Also, if the path to the music library is not visible, how can we manually update our music library in the future?? 

The answer is already given by John Guarr above - read back instead of just jumping in complaining! 

@ Greenland yeah I read that but it doesn’t help much once you’re at a different location where one has a seperate Sonos system that you cannot access because you didn’t bring your laptop and only have your iPhone available.

Well, go back and get your laptop, then .  At the moment, it's the only way to workaround the almighty mess of the updated apps on iThings and Android. 

Local library settings (like all other settings) are stored in the distributed database on the speakers. Nothing of any consequence is stored in the app itself.

That doesn’t explain why the app only randomly recognizes the music library.  The majority of the time the music library is missing, but occasionally the app shows it and I can play my music.

At the moment, you can only Manage the library settings using the PC versions of the Sonos program. 

Yes, that appears to be the only way to input the location of the library.  But using the desktop app to play music is not a suitable solution for using the Sonos products.

After you add the library using the desktop controller, if you close your mobile apps completely (or better yet, reset and then “Connect to an existing system”, the library should show on the mobile app.  The queue functions aren’t complete (fix schedule mid-June) but the library is there. 

Yes, that works for a bit, but a couple of hours later the music library mysteriously disappears from the mobile app.  It’s infuriating but since the music library always seems to appear in the web app, I’ve taken to using my iPad (the web app won’t work on the iPhone) as my controller.  Not perfect, but first world problems…