This guide explains how to restore the operating system on a Mac from a Time Machine backup. This can be used if the Mac is having problems starting and will restore files as to a previous time.Here is how to restore:1. With the Mac turned off, connect the Time Machine drive (e.g. external USB storage device) to the Mac.2. While holding down the Command (⌘) and ‘R’ keys on the keyboard, turn on the Mac. 3. In the macOS Utilities window, select Restore From Time Machine Backup and click Continue. 4. Select the Time Machine drive in the list of restore sources and click Continue. 5. Choose which backup to restore from, including date and time, and macOS version. Then click Continue. 6. Confirm the destination drive in the list of destinations. This should be the internal Mac drive. Then click Restore.