How can I adjust status based on EB-1C with an EB-5 conditional green card?

The timing is tricky but doable. Perhaps it is easiest with consular processing.

If you do not have valid nonimmigrant status, you are not eligible to apply for adjustment in the U.S. The smartest way to do these cases is to process the new case at your home consulate abroad and then surrender your conditional green card before you leave to attend that appointment. It has to be done by highly skilled professionals because if there is a snafu you can be stuck abroad.

A conditional lawful permanent is not eligible to adjust status in the United States. Therefore, you could process your immigrant visa application at the U.S. embassy or consulate in your home country. During that process, you would abandon your conditional green card and receive your immigrant visa based on the EB-1. Keep in mind that there must be an available immigrant visa number, i.e., priority date current, in order to receive your immigrant visa. You should consult an immigration attorney before you make any decisions or take any actions in this regard.