Now, if you click on the iCloud tab, you should see that your stuff is syncing and will be restored to your new Photo Library as soon as it’s downloaded, while anything you add will be uploaded. Remember back on our General tab there’s a “Library Location” option? Click “Use as System Photo Library” to convert the new library and enable the iCloud features. If you want to sync this library to your iCloud account, you have to first convert your new photo library to the “System Photo Library.” Now the Photos application opens to an empty library with ways to add photos to it: you can import them from a camera or memory card, import them from a location, or drag pictures directly into the application. Since you want to create a brand new library and populate it with pictures already stored on iCloud, click “Create New…”
Hold down the Option key on your keyboard. Make sure that Photos is not open (quit if it is). Fortunately, its easy to switch between multiple libraries. When you create a new library, you may still want to go back to a previous library for something. If you want to find an existing library in another location (such as the one you just backed up), click “Other Library…” How to switch between Libraries in Photos for Mac. If you have other photo libraries on your system, they’ll be listed here.