I want to know how I can upgrade my php using plesk?
I've done this using the atomic yum repository before. Instructions for yum are here: http://www.atomicorp.com/channels/plesk/
If course you need to be logged in with SSH as root to do this, no easy way that I know to do it from the Plesk interface. But I have done this on a Plesk 9.2 and a Plesk 9.5 installation before.
Edit: added sudo and apache restart in there
Now the atomic repo will directly install php 5.3 by default. So, just had a thought that this will be helpful for someone who is looking to install php 5.2.X from atomic.