You can upgrade the version of php on windows plesk server through the following steps
1) Download the php Binaries of the version which you want to upgrade
2) Stop the plesk services on the server
3) Goto the path D:\parallels\plesk\additional\pleskphp5
rename pleskphp5 to oldpleskphp5 and create a new folder named
pleskphp5 and extract the php binaries in it . Delete the php.ini file in it and copy the php.ini file from oldpleskphp5 and paste it in the new
4) Goto Server , Server Components and click Refresh .
Thats it , you will see the upgraded version of PHP