If an insurance provider finds that one of their customers has misrepresented something, they have several options. Which they choose depends on the nature of the misrepresentation.
Do nothing. If the misrepresentation was innocent and something small, the insurer can choose to treat the policy as valid and carry on as normal (though they’d probably at least correct the info).
Cancel the policy. The insurance provider can cancel the policy per the policy terms. Just as they would for something like non-payment, they can do so by sending notice of cancellation to the insured via registered mail, in which case the insured would have coverage until 15 days after they receive the notice.
Rescind the policy. In the most serious cases of material misrepresentation, the insurer can rescind the policy, meaning they basically treat it as though it never existed. This is known as treating the policy void ab initio.