hi, what Linux is yours? Here is what I've found:
"Plesk control panel itself doesn't use client PHP, its upgrade could affect only:
1. Site applications
2. Horde webmail
3. PPWSE and Sitebuilder"
There are no specific recommendations or steps because all the steps are identical to any other application update: 'rpm -Uvh' for RedHat-like OSes and 'dpkg -i' on Debians. Get package updates from OS vendor and use them. If OS vendor does not provide such packages, then you can try to find php v5 packaged with the third parties. There are hundreds of custom builds found all over the world, for example a well known
http://www.atomicrocketturtle.com has really been a great source for such updates for many years. The most of problems which may appear with it are usually related to dependencies to third or system applications and can be solved accordingly.
Also, SWsoft provides all Plesk packages necessary for php5 upgrade at
ftp://download1.swsoft.com/ (also see
http://autoupdate.plesk.com) with except of MySQL and PHP themselves, which are not the part of Plesk.
In fact, you can try to update the package in single transaction (if your mirrors are set correctly), for example:
# yum update php5.0.4... (RPMs)
# apt-get update php5.0.4... (DEBs)
Also try doing some search over this forum -- AFAIK there should be some similar threads with killograms of valuable info...