Tap on a category which you would like to edit or delete. Tap on the three dots icon at right top corner.
To edit name of the category tap on 'Edit' from the drop down list. A pop-up will appear, you can put new name of the category there and tap 'Update'.
To delete the category tap on 'Delete' from the drop down list. A pop-up will appear with confirmation.
If you tap on 'Delete' the category will be removed and the items in the category will be moved under 'Uncategorized' category.
If you also want to delete all items inside the category at the same time, just check the box 'Delete with all items in this category' and tap 'Delete'. The category including all items in it will be deleted.