How to update dependent variables manually..

Hi ,
Is it possible to update the dependent variables in each step manually in COMSOL ?
For example, I want to assign new values for a dependent variable (V) in each step like:
V ..... if (phils<0.5, 0, V)
BUT I always get Duplicate variable error. Because V is already defined in the solver.
Any ideas that how this is possible in COMSOL?

Thanks