To edit the Metadata of the file in Asset Library, follow the below steps. (Based on the Access Control)
Search and filter the file in Asset Library, then click on the Action button 
which is placed towards right to the bottom of the Thumbnail file.
Then you can see the Properties icons 
from the drop down options, click on this icon.
Now you can see the another Popup Menu contains file with Metadata values, click on the Edit option 
.
By using Edit option, you can delete and replace metadata value.
On clicking Save option 
you can add the new Metadata values. Now, the Metadata value got changed.
If you don't want to add anything then click on the Cancel option 
.
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