OS X El Capitan: View and restore past versions of documents
Many apps automatically save versions of documents as you work on them. At any time, you can browse through document versions and go back to an older version. You can also explicitly save a version.
A version is saved automatically every hour, or more frequently when you’re making many changes. A version is also saved when you open, save, duplicate, lock, rename, or revert a document.
Browse, restore, duplicate, and delete versions
Open the document, then choose File > Revert To > Browse All Versions.
Click tick marks along the timeline to browse versions.
Display the version you want, then do one of the following:
To restore your document to this version, click Restore.
To duplicate this version in a new document, press the Option key and click Restore a Copy.
To delete this version, move the pointer to the top of the screen to show the menu bar, then choose File > Revert To > Delete This Version.
To leave your document as is, without changes, click Done.
Explicitly save a version
Choose File > Save.
To save the document using a different filename, location, or format, press the Option key and choose File > Save As.
Revert a document to the last opened version
Choose File > Revert To, then choose Last Opened, Last Saved, or Previous Save.