insurance company is trying not to honor recently updated policy
Can they legally do this?
Based on the information you have provided, it appears that the insurer is entitled to rescind the policy update (I am assuming that the fire began prior to your request to update the policy, although this is not entirely clear from your inquiry).
That being said, knowing (1) the terms of your updated policy/contract, and (2) the details of your interactions with the insurer at the time of updating the policy, are vital toward identifying with more precision the merits of each party's position.
Your description reflects that the insurer is exercising its right to rescind the policy update. See DuBeck v. California Physicians' Service, 234 Cal.App.4th 1254, 1264-1265 (2015):
An insurer has the right to rescind a policy when the insured has
misrepresented or concealed material information in seeking to obtain
insurance. [...] When an insurance company, with full knowledge of all
the facts, enters into negotiations and relations with the assured,
recognizing the continued validity of the policy, the right to a
forfeiture for any previous default which may be asserted is waived.
(internal citations omitted).
Thus, your insurer would prevail if it proves your awareness and concealment of the fire at the time you sought to update the policy. This makes sense because, were it not for the concealment, the insurer would have adequate information for deciding whether to require a higher premium or decline your request altogether.
By contrast, you would prevail if you are able to prove that the insurer was (or should have been) aware of the heightened risk to which your property was exposed at the time the insurer confirmed that "the [policy] changes would take effect immediately". This is what the court alleges to have happened in the DuBeck case.
Your policy/contract might contain language that can be construed as equivalent to the insurer's pre-emptive waiver of its right to rescind the policy update(s), and/or language that formulates the notion of exposure in a way that favors your position. Hence the importance of knowing the terms of your policy/contract. Without knowing the details, it is impossible to guide you in that regard.