You should really check out/again the Beginners guide to C2, get used to expressions and check out the system expressions list.
You can directly use the name of your variable as expression.
If you want to get the value "123" out of "Variable1" and set it to "Variable2" for example:
Make a condition system set value global variable, pick "Variable2" and set value to "Variable1".
Variable2 will now contain Variable1's value.
This is covered and demonstrated in the beginner tutorial, and you can read more about variables in the How do I FAQ.
This article from Scirra's blog should be worth reading too.