There is a collection in the Collection Library that isn't quite what you need, and you don't want to spend time re-creating the entire collection from scratch.
While collections in the Collection Library can't be modified, it is possible to duplicate collections and modify the copies. This guide covers how.
This isn't required in order to modify custom collections but, since the Collection Library can't be modified, it is a requirement for editing collections from the Collection Library.
Any collection can be duplicated by right clicking the collection and selecting Duplicate or by selecting the collection and using the keyboard shortcut Alt+Insert. Duplicated collections will appear at the end of the list of collections in the tree.
Duplicating the lowest collection in a hierarchy will only duplicate that collection. Duplicating a collection with one or more child-collections will duplicate that parent-collection and all child-collections, exactly as they are configured.
Multiple un-related collections can also be selected at a time by using Ctrl+Click to highlight non-sequential collections or by using Shift+Click to highlight a sequential group of collections.
Right clicking any one of the highlighted collections and selecting Duplicate (or pressing Alt+Insert on your keyboard) will duplicate the selected collections.
Collections in the Collection Library do not use the Drill Down From Parent Collection option, so collection membership won't be affected by only duplicating a child collection to modify.
Duplicated Collection Library collections can be modified the same way as a custom collection, by double clicking the collection (or right click > Edit Collection) to open the collection for editing. Once opened, configure the required changes for this collection and click OK to save and close the collection filters.