SliQ Invoicing backup files are database files that can only be opened using SliQ Invoicing.
Each version of SliQ (V1, V2, V3, V4, V5 and so on) will restore backups created with older versions. For example V3 will restore data from backups created in V1 or V2.
To open a backup, go to the Setup
tab in SliQ and click the Restore
Alternatively, if you have started SliQ and no company is open, SliQ will show the SliQ Invoicing Plus - No Company Open
dialog. If this dialog is open, choose the Restore from a Backup
button to begin the restore process.
More help on restoring from backups can be found here