There are two ways to do global search and replace:
   use the / command to search for the target string
   use ONE change command (ex: cw for change word)

   use 'n' to find the next instance
   use '.' to repeat the same change

This is the safe way.


A more dangerous way is to use this:
  : RANGE s/SEARCH/REPLACE/FLAGS

Flags:
  g  replace all occurances on lines affected (not just the first on the line)
  c  ask for each replacement

Valid ranges include:
  %	whole document
  START,END a range of lines
  g/SEARCH/ all lines containing SEARCH

valid start and end line designations are:
  line number
  $   (last line)
  /SEARCH/   next occurance of SEARCH
 
For example:
  :%s/one/two/g

Note that this will also change 
   someone  into  sometwo
   hone	    into  htwo
Because of this, if you aren't sure, you should always use the 'c' flag.
Even so, it may still be easier to use the next and repeat operators
(above) because if you hit 'yes' one too many times, it is more difficult
to back up and continue the changes.