Default [Q] [HELP] Downgrade UMT

hello I'm french so I'll try to be a maximum understandable, but it is not easy with the translator
to start I have a htc desire hd 2.3.5 sense 3.0 and I want to Root, I understand that I must first downgrade but the Ultimate Multi Tool software is not functional, it does not work if I understand.
I want to know how to be able to root my htc DHD? because I want to provide Roms enjoy!
Thank you in advance for your answers
EDIT : up, i need help please :'(