Each update must be acknowledged and scheduled for installation by the user, although theoretically there might be a backdoor way for them to remotely install updates without user consent. If you ever go a Tesla Service Center you'll need to remember to tell them not to install software updates for you (since they will commonly do that as a courtesy).
However, there will also be a point where features in the older software versions may no longer be supported and capabilities may degrade, particularly if Tesla's server-side communications specific to those older versions are discontinued, particularly around the navigation features. The Tesla Service Center may also say that they are logistically unable to fix or support some issues without upgrading to a current version.
"The vast majority of successful major crimes against property are perpetrated by individuals abusing positions of trust." -- Lawrence Dalzell