If you are referring to the phpMyAdmin which the Plesk control panel uses, you would have to upgrade the version of Plesk. I have never kept track of what versions they use in each Plesk version, so I don't know if upgrading from Plesk 7.1.x to 7.5.x will be of use to you.
The version which they use in the control panel may have been modified, so I would not recommend messing with that version, or you might break the control panel. But if you try at your own risk, make sure you keep a backup handy. Plesk's version is kept in:
/usr/local/psa/admin/htdocs/domains/databases/phpMyAdmin
pay careful attention to their config.inc.php and how they have it setup!
If you are talking about a separate domain installation which is accessible directly by you/domainuser, then you just download, unpack the new version into the domain's directory where it was put in the first place and then remember to change the config.inc.php.