Change to normal mode with <ESC>.
Search (Wrapped around at end of file):
Search STRING forward : / STRING.
Search STRING backward: ? STRING.
Repeat search: n
Repeat search in opposite direction: N (SHIFT-n)
Replace: Same as with sed, 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