Answer
Usually a backup (.lpb) file can be restored under the Tools menu of Lightning Payroll. If the Restore Backup option is greyed out or unavailable then you are either on a trial licence, or unlicensed (your license may have expired). The option to create and restore backups is only available to users on a paid subscription to the software.
If you only want to restore the backup to view old data you can put Lightning into a read-only (unlicensed) mode. You will not be able to make any changes in this mode, but backups can be restored.
To switch from trial mode to unlicensed mode you need to delete the trial licence manually while the application is closed down. This file, called license.txt, is located in the following location depending on your OS:
Windows: C:/Users/Your User's Folder/AppData/Roaming/LightningPayroll/licence.txt
Mac: ~/.LightningPayroll/licence.txt
An easy way to get to the data folder to delete the license file is to click the ? on the lower left corner of the start up screen when you open the program, then select Open Data Folder.
Right click on the license file and delete. Please note,the below picture is for a Windows user.