Hi Affableaardvark !
The upgrade from v.1.1.5 to v.1.1.6 is relatively easy. Follow this link:
http://code.google.com/p/tomatocart-shoppingcart/downloads/list
and download update files v1.1.6.zip
In case you didn*t make code changes to your files that has to be updated
Unzip to some temporary folder, then upload all the files to your web shop to the same folders as in zip archive. When prompted to overwrite files, click Yes.
If you made changes to some of the files you shall make the update manually. If you have such files post them to me and I*ll do it for you.