You can't change this. You can add a reason.
I tried adding the reason, but it doesn't show up on the certificate. how can I make it visible on the certificate?
What can you see when you sign a document?
If you want to change the name of the name "JM" that appears here. That can be done when you create digital Id on Reader or Acrobat, it can't be altered once Id is created. And if this is purchased certificate then you have to check with Vendor.
For more information and changes that can be done are mentioned here: Custom Signature Appearances — Digital Signatures Guide for IT
Let us know if you have more questions.