How do I open or switch libraries?

Choose File › Open Library… and pick the library's folder.

Checked against Terra 0.1.0

Choose File › Open Library…, or press ⇧⌘O.
Pick the library's folder and confirm.
It opens in place of the current library, in the same window.

Shortcut: ⇧⌘O

Other ways

With no library open, click Choose folder.

Good to know

Any folder can be a library. Pick an empty one to start a new library.
Terra reopens your last library when it starts. Settings › General › Open at launch changes that.
To rename a library in the sidebar without renaming its folder, set Display name in Settings › General.
The Library menu at the bottom of the sidebar has Open in Finder and Copy library path.