How Do I Access The Previous Values Of A Variable?

It doesn't exist natively in the language C++ or C#.

To do what you want, the only way is to manually build a system that memorize your variable values.

The only practical usage I see is as a part of a debugger that would allow backward execution.

You cannot: memory is overwritten, previous values are lost.
If you need the previous values then you have to store them explicitely (e.g. using a collection, like for instance, a std::vector, or a plain array).
You may nicely wrap such a mechanism in your own 'Variable' class.

You can't.
Integers don't have a "history" that you can access, it's not like writing things down on paper and "crossing them out" when you have fished with them. It's more like taking a jar and putting five coins in. If you then empty it, mix all your coins together and put ten random coins in the jar you can't tell how many the jar had before.

Variables do not normally have a "history", except in very special cases when an audit trail is required, and then a lot of software goes on behind the scenes to log when changes are made, and by whom.

This is not possible unless you store the variable history yourself.
Variables is a simple storage location in memory not a DB with audit trails.