Substitutions are very advanced.

First a simple example:

s/apa/bepa/
This will substitue the 'apa' in current line to 'bepa'.
If an 'p' is appended, you will also immediately see the result.

1,$s/apa/bepa/
Same, but all lines in file. Only first occurence on every line.

Any character can used instead of '/':
s!apa!bepa!g
The 'g' specifies that all occurences of apa on this line are changed to bepa.

The pattern that are supposed to be replaced, can be a regular expression.
See ed3 about that.