Follow intructions carefully. I disclaim all liability for any harm that may befall your device. I'm simply posting a link to the information.
It also says that the OTA for this update should be available June 15th ! Personally I'm going to wait for the OTA so I don't have to wipe my device !
THIS WILL WIPE YOUR DATA!
http://www.androidpolice.com/2011/06/09/exclusive-htc-thunderbolt-mr2-update-1-66-605-2-links-to-stock-unrooted-custom-rooted-roms/