There are several ways to do this. Which one you would use depends on where the calculation expression exists: in the control source, in an underlying query definition, or in code.
Consider whether you should really do this, though. If the value can be calculated whenever it's needed, it's usually better to do that than to store the calculated value. If you avoid storing it, you don't have to worry about recalculating and updating if one of the values it depends on changes.
But if you're sure you want to do it:
Is the form bound to the table you want to update?
Where is the expression that calculates the value? Rick Sprague