Easiest way for an existing S-corp to change to an LLC?
If your business is registered in a state that allows entity conversions, it's typically some paperwork and state fees, along with an amendment with the IRS in regards to the company's federal employer identification number (EIN).
Unfortunately, not every state allows entity conversions. That being said, as every state, and subsequently every individual company's situation is respectively unique; it's hard to give you a direct answer without knowing the state in question.
Alternatively you could just dissolve the S-Corp and form a new LLC with a new name and tax ID.
I hope you are able to find this information helpful.