Using sed append function? Doesn't seem to stick.

[Decibels]

Not sure if this is the right place, but I wonder if something is wrong with SED or I am doing it wrong.

I finally got sed to append to the end of a line in a file.
Show the output, no errors, but it isn't really writing it to the file.
I am trying to make a patch, but the text I am trying to write just isn't
actually appearing in the file after.

[Decibels]

I figured it out. Apparently for SED you can delete,... lines in a file, but
to append or insert a line you have to write to another file and then copy
the file over the old one.