2.9BSD/usr/lib/learn/editor/L30.2a

#print
Changing a whole line with the 'c' command is a slow way
to work when you only want to change a few letters.  The 's' (substitute)
command changes the letters within a line.  For example, the command
  s/abc/def/
will change a line reading
  xxxabc
into one reading
  xxxdef
by replacing the string 'abc' with 'def'.  Try the following sequence which
alters the first line of the file 'test' within this directory:

ed test
1p
s/months/years/
1p
w
q
ready
#create Ref
Four score and seven years ago our
fathers brought forth ...
#create test
Four score and seven months ago our
fathers brought forth ...
#user
#cmp test Ref
#log
#next
30.2b 5