How can I change the access priviledges from a workstation?

You can change only password.

To change access, you must be the only host e.g. you must open database on your Machine with administrator privileges.

Do you mean to say that in order to change passwords, I have to copy the files from the server to my local drive, and then afterward copy them back? How can I do this? My computer is a pc, the server is a mac...?

Shouldn't be a problem. Filemaker files can be opened on either a Mac or PC as long as it's running the software!

I believe that if you host the files with FMP Server you can do all of this remotely as long as you are the only user. (Ie get the others to log-off.)