Hi,
If I understood your query correctly, you're looking to sort the lookup column in your document library. If you would like to modify the existing view only, you can select the header of your lookup column and select Descending and then you could save the current view for it to persist. If you would like create a new separate view with Desc sort configured of your lookup column, you can create a new view and configure the sort as per your need.
Please refer to this link which will help you in locating the sorting configurations in views and you could select your lookup column as per your need -
https://support.microsoft.com/en-us/article/use-sorting-to-modify-a-sharepoint-view-1e85d6e7-bdb3-41...Hope this helps.
Thanks.
