The new version of Collector (Collector of ArcGIS) does not have a tool to delete imported .TPK basemaps like Collector Classic did. Instead, users must connect via iTunes File Sharing to delete imported basemaps as described here: https://community.esri.com/message/839011-how-to-delete-basemap-tpk
Please add back the ability to delete imported .TPK basemap files from Collector for ArcGIS in the app itself.
(Though I admit that it is possible to delete the folder containing the .TPK file you wish to delete from the iOS Files app. -- Just open the Collector "folder" in the Files app, open the Basemaps folder, and open each folder until you locate the .TPK basemap file you wish to delete. Delete the folder containing the basemap file. -- This was easier in the Collector Classic app.)
Adding the ability to import .TPK basemaps from the iOS Files app is awesome! Especially when importing from a flash drive without the need for a broadband connection. (As of June 2019)