I have accepted (and signed) an offer from company A (I should start working in 2 months).
After 2 weeks I got a proposal from company B. Proposal B is much better for my career-path and for my future employability.
How would you tell company A that you have changed your mind and will not start working there?
p.s. I know that this act is not "ok", but my career and future opportunities are far more important. It would be not ok to my family to take offerA, and my family comes first!