You need to look at, and follow, the terms of the trust agreement. The document may spell out exactly how you can change the beneficiaries. Some agreements are silent, in which case, you probably can't change the beneficiary. Some of them permit change only by will. Others may say that you may change the charitable beneficiaries in an document executed with the formalities of a deed. The trust agreement may also state to whom the document effectuating the change should be delivered.
You should probably speak with a qualified estate and trust lawyer. If you don't know one, you can get a list of competent practitioners from the American College of Trust and Estate Counsel.
I hope that this helps.
I agree with the above answer. You change bene's by following the terms of the trust. If the trust is silent on how to do it, but does allow you to amend, then a trust amendment should be used, executed in the same manner as the trust. A short visit with an estate planning lawyer should get it done for not too much money.