Can I change the beneficiaries of my revocable trust by writing the changes and having it witnessed and notarized?

While a court may approve that sort of amendment, I would avoid it as it is likely to be challenged (and who wants to put their heirs through an unnecessary court proceeding?).  Better practice would be to do the amendment separately - and do get it notarized.

Yes you can change the terms of your living trust at any time.  You need to prepare an amendment to your trust and note the changes to the trust in the amendment.  Once you prepare the amendment, you will need to have the amendment witnessed and notarized.  The signed amendment will make the changes permanent.