:%s/oldstring/newstring/gc
This command will replace all the occurences for oldstring with newstring after confirming for each existence (c in the end tells vim to ask for confirmation).
More on search replace – http://vim.wikia.com/wiki/Search_and_replace
Some other useful commands are
/ for search n find next while search (after /) N find previous while searcg (after /) :$ last line of file :0 first line of file