1. Quit Photos.
2. Hold down the Option key and open Photos.
3. Select the library you want to open. If the library you want isn’t listed, click Other Library. Navigate to the library you want, and click Open.
4. If the library you want isn’t listed, click Other Library. Navigate to the library you want and click Open.

You can also drag the library you’d like to open to the Photos icon in the Dock. Or you can click Create New to create an all-new, empty library.

Photos can use libraries from iPhoto 8.0 or later and any version of Aperture. If you’d like to migrate a library from iPhoto 7 or earlier, use Apple’s free iPhoto Library Upgrader tool to prepare your library first.

About your new Photos library

• Photos stores your library in the Pictures folder on your Mac by default.
• If you upgrade an iPhoto or Aperture library, your Photos library will be stored in the same location as the original library. Photos uses the same master files as iPhoto or Aperture, so you won’t need double the space to run both apps.
• Photos turns iPhoto Events into albums, which you’ll find in a folder called iPhoto Events in the Albums view.
• Some metadata, like star ratings and flags, will be transformed into keywords, so you can easily search for them. Learn more about how Photos handles Smart Albums and content and metadata from iPhoto and Aperture.
• If you’re using multiple libraries, you might need to designate a System Photo Library, so iCloud and other apps, like Mail and iMovie, can access the photos you want to use.

 If you store your Photos library in a location that’s part of a cloud service (for example, Dropbox, Box, or Google Drive), your library might be damaged by the syncing process. As a result, you could lose photos.
To see your photos on all your devices, you can use iCloud Photo Library. To back up your library, you can use Time Machine.

Using iPhoto and Aperture after you migrate to Photos

iPhoto — and Aperture, if you downloaded it — remains on your system in the Applications folder, and you can continue to use it. If you open iPhoto or Aperture after you upgrade to Photos, you’ll be asked if you want to open your library there or in Photos.

Any changes or edits you make in iPhoto or Aperture aren’t reflected in Photos. And any changes or edits you make in Photos will not be reflected in iPhoto or Aperture.

Turn on iCloud Photo Library to access your photos on other Apple devices

If you’d like to use iCloud Photo Library after you upgrade to Photos, you can turn it on in Photos > Preferences > iCloud. Photos and videos from your System Photo Library will begin to upload to iCloud in their full resolution and original format. Photos will estimate the size of your collection and help you select a different iCloud storage plan if needed.

iCloud Photo Library will automatically manage the size of your library on each of your devices when you choose to optimize your storage in Photos > Preferences > iCloud. Learn what you can do if your photo library is over 1 TB.

iCloud Photo Library keeps a single library — your System Photo Library — up to date on all your devices. If you change your System Photo Library, the contents of this new library will be merged with the contents already in iCloud. This can’t be reversed, so you should make this change only if you intend to combine your libraries.

Use Time Machine to back up your Photos library

Your photos and videos are important. Even if you use iCloud Photo Library, it’s recommended that youuse Time Machine to back up your Mac, including your Photos library.

Keep your Photos library and Time Machine backups on different disks. An easy way to do this is to keep your Photos library in your Pictures folder (the default location), and set Time Machine to back up to an external drive, a disk on your network, or a Time Capsule.