How can I change all occurrences of xxxxx to yyyyy within vi?



To change only the first occurrence of something (per line) in the file, do this:
<esc>:%s/hsots/hosts/


To change every occurrence on each line of the file:
<esc>:%s/hsots/hosts/g


To change all occurrences of hsots to hosts on lines 3-5, do:
<esc>:3,5s/hsots/hosts/g



Yeah, yeah, you get the idea....