Christoph Kling wrote:

I created a new contacts "folder" in my exchange mailbox and need to
delete the old (default) one, but the appropriate button is greyed
out.

You can't delete default folders.

My question is how I can bypass that and how I can make the new
folder the default for all uses in outlook.

Not possible.

I created a new folder
because I was unable to rename the old folder (same reason: button
greyed out). So if somebody knows how I can rename the default
folder, my problem was solved, too.

You can't do that, either. State you goal and maybe someone will be able to
suggest an alternative approach.
--
Brian Tillman