What should I do if grandmother is experiencing severe manic episodes?

Severe uncontrolled. mania warrants urgent medical intervention. Please contact her psychiatrist or consider ER.
Get her to doctor. Best thing to do is to help your grandmother get medical care for these "severe manic episodes." if she already has a psychiatrist, that doctor needs to be called -- and can suggest options of either medication adjustments or inpatient hospitalization. If she is severely ill right now, she may need at least a brief inpatient stay to stabilize. Good luck to you.
TREATMENT. Grandma has to want therapy and medication, and all you can really do is encourage her to seek both and engage deeply and meaningfully in the therapeutic process. You simply cannot do it for her, much as you'd like to. You can either be her grandchild or her therapist. You cannot be both, and even though you want to see her get better, trying to be both simply will not work.