Help, I just found a letter saying I had to go to jury duty four months ago

I just found the letter in a stack of old mail. What are my options? Do you I have to show up in person to the court house to reschedule it? Or do they automatically do it? I also have some things that might limit me from attending jury duty. Namely Bipolar Disorder and delayed sleep phase disorder. Though I'm not sure if that qualifies as an excuse. What should I do?