Evaluate the risk to your loved one's safety. If the risk is high, step in; if not, step back. If you are unsure whether to step in, step back and get your bearings. If you can't evalutate the risk, get professional help. You might be too close to the situation to be objective. If so, hire a neutral third party - a care manager - to assess the risks and advise you.
Personality changes don't occur in old age without a cause. You cannot talk someone out of being suspicious, but you can support whatever they are feeling. Whether they are scared, unsettled, or upset.
Try humor as a way of responding to controlling behavior. Sometimes the best approach is to be direct, but with gentleness and reassurance. Don't become controlling yourself to try to make your loved one agree with you.