The indev branch will be for the latest Gnomoria changes. There will be more frequent updates here but also likely more bugs. As these updates get fleshed out, the build will periodically get copied over to the default branch and updated everywhere else.
Before opting into the indev branch, make sure to back up your saves! There will be a chance here of some updates irreparably breaking your saves. To back them up, travel to "My Documents" or "Documents" and then My Games/Gnomoria/Worlds. You can back up individual saves or just copy your whole Worlds folder.
To opt into the branch right click Gnomoria in your Steam library and select "Properties"
Click on the "Beta" tab and select "indev" from the dropdown.
If you don't see "indev" you might need to restart the Steam client. If Gnomoria doesn't update after changing branches you can try either restarting Steam or in the "Local Files" tab choose "VERIFY INTEGRITY OF GAME CACHE"