Search and Replace on VIM

1. Search and Replace All (Easiest)

Esc
:%s/old_pattern/new_pattern/g

2. Search and Replace selected

(a) Esc
(b) /old_pattern
(c) Go to desired by pressing n again and again
(d) Press cw (Change Word) and type
(e) Esc
(f) Press n to go to next old_pattern
(g) Press . if you want to do repeat last command used in step (c)
(h) Go to step (f), until all desired changes are made