I opened in SETUP the UNIT OF MEASURE. Here I want to delete some UoM’s which I surely do not need. But it refuses with the message : Cannot delete or cancel because UOM Tesla is linked with UOM Conversion Factor MAT-UOM-CNV-00231
This is usually a link which will take you the linked document.
On the other hand, if you open any document (in your case the UoM), and Menu > links will list all the links to that particular document. You can delete the links if you are sure those are irrelevant before deleting the document.