How can I set the label of multiple items at once?

You need first to enable the advanced feature:

Then you save and refresh the client side. After that, in any folder in which you want to change the labels, you can select the option “Change Labels” under tools:

You make the selection of all the items needing a label change, select the label and click on SET or UNSET:

All the selected items will have the Label2 SET if SET button is pressed.