you should be able to edit the/a page view -> add a resource and look for "edit custom property value" or something similiar. It should then give you a list of properties you want to edit.
Some fields can't be edited. The Contact field you are talking about is a field populated by SNMP and can't be edited directly. Any change you make would be overwritten eventually. Suggest you create a new CP similar to Contact and then you can edit it. Otherwise edit the contact information on the device directly.
That makes sense, now that you mention it.