How to find and replace text in Vim

:%s

Type :, then s/old/new/, then Enter. That replaces the first old on the current line.

Add g at the end to replace every match on the line. Put % before the s to cover every line: :%s/old/new/g replaces it everywhere.

Worked example

Replace every draft in the file with ready.

:%s/draft/ready/gEnter

Your textNormal

One u undoes the whole replacement, in every line.

:s/old/new/: Replace the first match on this line.
:s/old/new/g: Replace every match on this line.
:%s/old/new/g: Replace every match in the file.
:2,3s/old/new/g: Replace on lines 2 to 3 only.
:%s/old/new/gc: Ask before each one. Press y to replace or n to skip.
:%s/\<old\>/new/g: Match whole words only. This works in Vim; this site's editor does not support \< and \>.
u: Undo the whole replacement.

The old text is a pattern. A dot matches any character, so :s/1.5/x/ also matches 125. Put a backslash before the dot, 1\.5, to match a dot only.
Without the backslash, the dot matches the 2 in 125 first.
Without g, only the first match on each line changes, even with %.
When only some matches should change, decide one at a time instead: search, change one match with cgn, then press . for each match you want. See the guide to repeating changes.

The course practises the careful way: search, change one match, look at the next, and repeat only where it belongs.