Assembly variants are a simple idea - you have one physical PCB but can vary the components that are fitted in terms of a parameter value (e.g. capacitance, resistance) or fitted/not-fitted. Setting up variants and changing the values is very easy using the variant management dialog accessed from Project | Variants. However using the variant manager does not provide a method to reset a parameter back to default.
Take the following as an example. C300 will be edited to be 10.2uF in variant A and not fitted in variant B.
The difficult case is how to revert variant A 10.2uF back to the component default. You can just type in 10uF as the value for A but that is still an assigned value and it will not track the original component value. To undo the varying of C300 we need to visit the schematic, select the second editor tab (U_Power) and then double click or Right Click > Parameter Variations. From the Parameter Variations dialog untick the Varied Value checkbox to restore the default value.
Really easy once you know how 
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One extra tip. If the varied value was a blank (empty string) then you will struggle because an empty parameter cannot be right clicked to invoke the dialog. To work around this use the Variant Manager dialog to set a dummy value for the parameter that can be clicked on.