If you wish to do so, you should take the following steps:
1. At the top of your Dashboard page, hover your mouse over the Reports tab located at the top of your Dashboard page to reveal the drop-down menu, and then click on the Custom Reports option.
A list of all your existing custom reports appears.
2. Find the custom report you wish to add a label to and click on the plus icon located on its' right, under the Labels column.
3. On the pop-up window appearing, type in your desired label name and press Enter or just click on the + icon. Keep in mind that you can add more than one at the same time and you can also use one of your existing labels, as they are available inside the pop-up window. Then tap on Save labels to add the label(s) to your custom report.
4. To remove a label from your custom report, simply click on the - icon next to the label name.
The label will instantly be removed.