To make changes in DNS Zone, please follow the following steps :-
1) Login to your Plesk control panel.
2) Click on "Subscriptions".
3) Click on "Manage Hosting".
4) Click on "Websites and Domains".
5) Click on the DNS Settings.
6) Click on [Manage] in the front of that domain name only.
7) Click on TXT DNS record for modify. i.e. marked in RED
8) Replace the TXT record with new one.
9) Click Ok
10) Click on "UPDATE" to apply the changes.
Information: Changes made to DNS records were saved.
