I have one weak password for many of my websites. Is there any way to change this one password for all these websites at once (or enmasse)? Or do I have to change them one-by-one?
One by one is needed.One of the ideas behind 1PW is that you cam - and should - use different passwords for different sites.
It's not feasible for 1Password (or other applications) to make password changes on several actual web sites at the same time. In other words, changing passwords directly in 1Password Login items won't make the password changes on their corresponding web sites (with differing methods of changing passwords).
1Password item details can only be changed one item at a time. We've had requests to support batch/bulk item editing and it's under consideration.