Upgrade php 5.1.6 on my linux virtual server

I have a virtual server with 1and1 and want to install Joomla 1.7 which is available from the plesk panel but it won't allow me because the php version needs to be 5.2.0 or greater, while current is 5.1.6. I therefore need my php upgraded. Not sure if this is relevant or not but I have a Joomla 1.5.23 site and two tikiwiki sites already running on the server. If these need amendment as a result of the upgrade then I would like to have the cost for this work included in your estimate. By the way, I do not consider this work as urgent - so I am happy for you to work this around other project work. Many thanks