Hello,
There are two things you can do.
To become the project owner, you can go to the WEB Access
tool. In project center, highlight the project that you
wish to own and click "Edit Project Details". You can
change the project owner here.
This however will not make you the project manager of all
of the tasks.
To become the project manager of a task or of all tasks
you must open the project in Project Professional and
click Collaborate->Publish->Re-publish Assignments
then check the box that reads "become the manager for
these assignments"
Good Luck
Regards,
Tom Romeo