Hi Neenu,
You mean that if you have A and B, A is checked, and the user checks B, you'd like B to stay checked, and A to become unchecked? I'm afraid you can't do that in a declarative way using expressions in the Form Builder UI. This means that users will have to uncheck A before they check B. Hopefully they can live with it.
Alternatively, you could use radio buttons, and have a last (or first) option that says "None of the above/following", or "No selection made", or something similar. This is a more traditional way of solving this issue, and also the solution that I would recommend.
Alex
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