You probably can't.
Normally, the seller can send you a 'feedback revision request' which comes via an email from Ebay and you respond to that.
However, the seller is already showing 5 feedback revisions on their feedback page and I believe that is the limit a seller can have during a set period of time.
It states this on my account: 'You are eligible for 5 Feedback revision requests per 1,000 Feedback received in the past 12 months.'
So, I think you cannot change it in this case. The seller has already persuaded at least 5 unhappy buyers to revise their feedback.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)