go to -> Domains -> website -> Dashboard -> SSL/TLS certification
Top left of the page press "reissue certification"
Down on the page click the "install a free basic certificate"
Make sure to select all, except "Dane"
On the next page copy the DNS code.
Go to the clouflare website, look up the site your are reissuing the certificate for.
go to the -> DNS -> Records
open the _acme-challenge TXT, delete the old string and add the new string.
It will save automatically.
When this is done go to the MXtools website, and check if the TXT has been updated to the new value.
You do this by entering the website, and select in the scroll down menu "TXT Lookup"
if this is updated or gives no issue's you can go back to plesk and press continue on the page.