Labels are an informal way for you to categorize your initiatives so that you can apply a filter and see only the initiatives that match your selected label. You can read more about how to apply labels in this article - Using labels to tag and filter initiatives.
If you need to rename or delete a label that is assigned to lots of initiatives can do so in two ways;
rename or delete a label for an initiative in its properties panel, one at a time; or
manage all labels for all initiatives in the canvas properties, which will apply a bulk update to all initiatives automatically.
Managing Labels for the whole canvas
Open the Canvas Properties by clicking on the pencil icon in the top right corner. Click on the Labels tab to see all labels (as shown below).
When you rename a label in the Canvas Properties, then all corresponding labels assigned to your initiatives are automatically renamed.
When you delete a label in the Canvas Properties, then all corresponding labels assigned to your initiatives are deleted.
Note, you cannot add a new label in the Canvas Properties window because a label must be assigned to an initiative.