Hi Scott
I have gone through your suggestion as follows:-
1. Upload my shapefile boundary as a hosted feature layer.
2. Upload my CSV file as a hosted table.
3. Create a web map with both layers and then use the 'Join Features' option with 'Create results as hosted feature layer view' ticked which produces a new hosted feature layer view.
I then go back to my hosted CSV table and look at the options to try and overwrite it, but I only seem to have the option to append as shown in the attached image.
I wish to overwrite my CSV table as all the data will have changed, are you able to offer any suggestions as to why I do not see the overwrite option?
Thanks