How to transfer the US-based 401k account balance to a 2a or 3a retirement planning account in Switzerland?

The company I worked for in the U.S. is closing down the 401k plan. I moved back to Switzerland and would like to transfer the funds from this US-based 401k account back to a retirement planning account (so called 2a or 3a) without incurring taxes. HOW DO I DO THIS?

You cannot. That 401(k) is pretax money and sooner or later you will have to pay U.S. income tax on it.