EventsClassic: How to move Non-respondents to another status.

The Non-respondents list, is a live dynamic record for contacts that have been sent an email linked to an event , and has not registered, declined, overbooked, or cancelled. 

Since the non-respondents list is more like a report, you cannot edit them manually like you can do with another another statuses. The invitees will need to respond via clicking Accept or Decline button from your email, so they can move to another corresponding status.

If you don't have a Decline button, or your recipient is responding to you another way, instead of responding to your email via button click, you will need to add them manually as registrants first using Ad Hoc and input the exact details (case sensitive) of their First Name, Last Name and Email address that are showing in the non-respondents list.

Once they are added as registrants, from there, you can move the status.