Same way you'd overturn any civil right. Get a two-thirds majority of states to agree and hold a constitutional convention. Probably easier to just move to a less free country.
I understand that to you, freedom means the ability to carry a gun around and decide when another person should die (i.e. at what threat level shooting them is justified).
But please don't be so dismissive - freedom to me means not having to worry about my kids being shot at school, or having to carry a gun around to defend myself with.
Other countries you describe as "less free," I'd describe as "more free."
Freedom is the absence of coercive interference over one's life. Exercising self-defense with adequate tools is affirmatively more free than being forbidden from doing so.
Safety is not freedom. Both are desirable but conceptually there is no overlap, and in reality there is often a tradeoff. Conflating the two as though safety is a form of freedom is a pretty good shibboleth for authoritarians; let alone defining freedom as the feeling of safety, which is textbook doublethink.
The US has strayed in many ways, but we seem to have the most people out of anywhere who are still invested in actual freedom.
