First you need to physically copy the picture library to the new location, and then you have to tell Photos where the new location is. Here's how to do this:
• Go to the Pictures folder (in your home directory) and locate the "Photos Library" package
• Copy the "Photos Library" package to the new destination by dragging and dropping it to the desired location
• Hold the Option (Alt) key down and launch Photos
• Find the Photos Library in the list or click the "Other Library…" button to manually navigate to the new location and then click the "Choose" button
Now Photos will use the library you have specified in the new location. Be aware that if the library is located on an an external hard drive and you launch Photos without that drive connected, your images won’t be available.