Note: If the database module is not displayed, open it with the ribbon command Mailings | group Recipients | Select recipients.
In the database module, you will find the command Select 
| Current record in the group Records to select or deselect the data record currently displayed in the database module.
If you choose this command for an unselected record, it will be selected and vice versa. If you choose this command for an already selected record, the record will be removed from the selection.
Tip: For a better overview of the records, switch to the list view (using the Form/list button in the group View). Here, you can also select/deselect records by enabling/disabling the checkbox in front of the record.
With the commands Select | Select all records and Select | Deselect all records, you can select or deselect all records of the database at once.
The command Select | Invert selection deselects all selected records and selects all unselected records.
Display in the status bar: The status bar at the bottom of the window also shows whether a data record is selected or not: The word Selected appears here for a selected data record.