Vi : Search and Replace

Change to normal mode with escape key.

Search:

Search STRING forward :
/STRING

Search STRING backward:
?STRING.

Repeat search: n
Repeat search in opposite direction: N (SHIFT-n)

Replace: Replace OLD with NEW:

First occurrence on current line:
:s/OLD/NEW

Globally (all) on current line:
:s/OLD/NEW/g

Between two lines #,#:
:#,#s/OLD/NEW/g

Every occurrence in file:
:%s/OLD/NEW/g