The only way is to make a new encounter and sign it. Then the allergy section will change. You could generate an encounter and simply put "allergy update" in the CC and sign it.
I WISH WISH WISH adding allergies would be as simple as adding meds and problems through the summary sheet.
Barbara, If you double click on the medication section of the encounter, you can add meds without them showing up in the plan portion. BUT if you change them in the summary sheet or from th RX button, they will show up in the plan.