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Navigate to the Subjects module. Click the Merge Subjects link in the sidebar.
Select the subjects that are to be merged. These are the source subjects. You can select multiple source subjects by holding the shift or alt keys on your keyboard.
If you would like the source subjects to be deleted after they are merged, check the respective checkbox.
Select the target subject. This is the subject that the source subjects will be merged into.
Click the Merge to perform the merge.