[Forum] change the password of vBulletin by another system

[pedro.bit]

Hello,
I'm building a web dynamic web site that have an its users management system.
I've placed side by side vBulletin to my site.
I made that when an user registrer heself in my system automaticaly he is registred in vbulletin also.
I made that when an user change his email address in my system automaticaly it is changed in vbulletin also.

How can i made that when an user chage his password in my system it is changed in vbulletin...

please help me!!!!!!!