To replace text in a buffer, you can use the :s[ubstitute] command in the command line. Its syntax is:
One example is:
This would replace every occurrence of foo by bar in the whole document.
Let's check the different parts of the command syntax:
- Range ([range])
It defines in which section of the document the substitutions will take place. The options are:
%: whole file
N,M: between lines N and M (eg. :145,160s/foo/bar/)
'<,'>: current visual selection. You don't have to type this yourself. Just select the lines you want to perform the substitution in and press :, these makers will be added automatically.
(nothing): current line
- Delimiters (/)
The delimiters mark the different parts of the command. The character most commonly used for this is / but actually you can use some other like _ if either your pattern or the substitution string contain slashes themselves (although you can also escape the slash using \/). For example:
- Search pattern (<pattern>)
The pattern to look for is a regular expression. The syntax is the same as described in the Searching section. Also, like in searches, it is recommended to use the very magic syntax, by prefixing the search pattern with \v, if you need to use special characters in your expression:
start of line
end of line
repeated zero or more times
repeated one or more times
repeated zero or one time
repeated N times
repeated between N and M times
any character specified inside 
any character not specified inside [^]
define group (for example to separate with |)
end of line
a digit (ie. [0-9])
a letter (ie. [A-Za-z])
whitespace (ie. [\ \t])
will replace Hello and Hallo by Bye.
- Substitution string (<string>)
All the matches of the provided pattern will be replaced by <string>. This string is not a pattern, which means that you can use any symbol directly in it without having to scape it.
Inside the string you can reference the regex groups that are defined in the pattern with () using the number of the group as \1, \2, \3... (\0 refers to the entire match).
For example, the command:
:s/\v^Hello ([^\s]+)!$/Bye \1!/
- Flags (flags)
You can add one character flags to the end of the command to modify how the substitution is performed. Some of the most important ones are:
g: substitute all occurrences in each line. Without this flag only the first occurrence of each line is substituted. Most of the time, you want to include this one.
c: confirm substitutions. For each substitution, it interactively asks if you want to perform the substitution or skip (at each occurrence you can also choose to stop substituting and substituting all the rest of the occurrences).
i: ignore case when matching the pattern.
You can add any combination of these. For example:
will substitute foo, Foo, FOO or any combination of upper/lower case letters in the word by bar in the whole document —all occurrences in each line— but asking for confirmation before each substitution.
To know more...
The options listed here are arguably the most important ones. However, the :s command features more than these. Execute :help :s to get a full description of the command.