You don't.
There should be no problem with the updated version.
If there is, it is most likely caused by something else.
Upgrading to Direct X 9 is a one way trip.
Unless it was recent enough you can use System Restore.
Start/All Programs/Accessories/System Tools/System Restore
Otherwise a Repair Installation may leave the original version of
Direct X.
Repair Installation:
http://www.dougknox.com/xp/tips/xp_repair_install.htm
Windows Updates will need to be reinstalled.
Data should be safe, back-up important data just in case.
Install Service Pack first and by itself.
Then install ALL Critical Updates no more than 4 at a time,
Disconnect the network/modem cable before starting repair and do NOT
reconnect the cable until repair is completed and firewall is
installed/enabled.
http://support.microsoft.com/?kbid=283673