How do you turn down a friend who is trying to sell you an insurance product?

Can say something like, "I already have existing servicing agent(s) and have no intention to change to another. Thank you.".

As a person that tried to sell MLM to friends before (about 1 month before I realise I not cut out of it).
My Friends just told me, “sorry ah but I really not interested.” And we’re ok.

Just firmly and politely turn him down. Don’t say ambiguous things like, “I’ll think about it.” You’re just asking him to check back with you again.

I’m not sure how far he has gone to sell the product to you so don’t want to suggest more stronger method to reject.

If a 'friend' shows up out of the blue after months or years of not keeping in touch with the sole intention to sell you an insurance product, I would re-evaluate my relationship with said friend.

But jokes aside and if he's a good buddy, then it makes sense to listen to him.
Even if you are not looking to switch as he's said, you can look at other ways to support him for eg. helping him refine his sales pitch and recommending him to your friends, provided you feel that he has his clients best interests at heart.
I have an insurance agent that sends me the occasional message on, hey what if you get an accident/die/etc etc - do you want to buy this additional coverage. Needless to say, I have bought other insurance policies from more well meaning insurance agents.

Just be honest and say you don't see a need to switch now, but appreciate his thoughts and that you'd keep him in mind if you decide to switch one day! :)