Label migration involves merging one label into another across all the issues associated with a project.
To migrate label:
Select to labels from the table on which you want to perform operation

Click on “Migrate” button from the dialog-box appear

A pop-up dialog-box show, where you can choose which label to migrate, choose the target label.

Click on “OK” to start the operation, this will show a progress bar progressing on each issue update.

After Completion of operation refresh the page to see the result (update the issue count of label).
