Download the OTA update.
Multiupload.com - upload your files to multiple file hosting sites!
Put the downloaded .zip on your microSD card.
Boot into recovery mode by using Quick Boot (my preferred method) or by turning off the EVO, powering it back on while holding the volume down button, and selecting the "recovery" option from the list.
Select "install zip from sdcard" -> "choose zip from sdcard" and select the file from wherever you put it in step #2.
Wait for the installation to complete, rebooting manually if necessary.
Use unrevoked 3.21 to re-root your phone.
I can't take credit, got this from "Good and EVO.net" It worked for me!