Once QSOs are selected for QSL or address label printing, you can "right-click" on these QSOs to drop down the context menu. Continuing to File > Print... > Options

You will see the following options for actions that will be taken when you print QSL labels:

Enable Automatic QSL Sent To the Following

This enables you to automatically mark the QSOs, that you printed labels for, to be updated to indicate that you have sent them. 

[Note: While there are several values of Sent, the appropriate value in this case is "Yes", if you enable this automation. You can choose via Direct, Bureau, Electronic (which does not make sense in this case), or Manager.

Sent the Date to Print Date

This enables you to update the "QSL via" for the QSO records you are printing labels for.

Set Via:

This enables you to update the QSO manager for the QSO related to the QSL labels you are printing. While this feature exists, it should not be used unless ALL the QSL labels being printed are for the same callsign. It will update the QSO manager for all QSO that QSL labels are printed for.

[Caution 1: This is not meant to provide a place where you can enter your own QSL return route.]

[Caution 2: If you use this for more than one QSO, ALL the QSO records will have its QSL Via (Manager) field updated.