Press and hold the Option key and click the Photos icon in the Applications folder (or click the Photos icon in the Dock).
In the Choose Library dialogue, do one of the following:
Click the library you want to use, then click Choose Library.
If the library you want doesn’t appear in the Choose Library dialog, click Other Library, navigate to the location of the library, select it, then click Choose Library.
You can switch to another library and designate it as the System Photo Library.
Restore a photo library from Time Machine on Mac