Plugin to insert content that I can update later in every post?

I want to be able to context, text, images, etc, that I can insert into every post (I don’t mind manually pasting in some string if need be) from a template, so that when I update the template, it will update all the existing posts as well without me having to go through an manually update them all.

I tried searching the forum first, but it’s kind of hard to know what strings to search for. Hope you can help.