Updating existing values in a list

Is it possible to update an existing value in a list? For example if I wanted to append text to an existing string value that I had previously added to a list is that possible? I'm able to extract the existing value (get() function) and append the additional text, but can't find a way to add the new value back to the list in the same index location. One work around I found was to delete the existing list entry and then add the new value, however this changes the index location to the last entry in the list - ideally trying to keep the index location static.

Thanks