Hold down the Option key and double-click the Photos icon in the Applications folder (or click the Photos icon in the Dock) to open Photos.
In the Choose Library dialog, 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. For information about considerations to keep in mind when choosing the System Photo Library, see Change where your files are stored in Photos on Mac