I often forget the arcane commands of the Vi editor. Here's one that is useful:

 (escape) :% s/strin/strout/g

That will sUBSTITUTE all instances of strin with strout in your document (g is for global -- without it only the fist strin is changed). That's all. If you are looking for a complete Vi reference, there's a decent book by O'Reilly on the topic.