Change second trustee

Quick question, my trust account was approved at PNC however my second trustee is incarcerated and I am currently his POA however I don't believe I can sign for him being as though the paperwork was already signed prior to approval. My question is, to change the second trustee I was told I would have to do a letter of resignation stating for old trustee to sign off and new trustee would sign? Where do I find a copy of this resignation letter or an example copy?