Changing password for pages site wide

I have a lot of pages with passwords and want to change them site wide once a month to stop people getting access to the information if they have not registered. I dont want to have to go into each page and do it manually.

Is there are plugin or something I can do in the data base to do this easily without changing the usual password to login to the account etc. Actually if this changed to to I dont mind but would like a simple function to make it happen.

Quentin