sed: how to replace line if found or append to end of file if not found?

-------------------------------------------------------------------------------
Become or hire the top 3% of the developers on Toptal https://topt.al/25cXVn
-------------------------------------------------------------------------------

Music by Eric Matyas
https://www.soundimage.org
Track title: Mysterious Puzzle

--

Chapters
00:00 Question
00:57 Accepted answer (Score 54)
01:49 Answer 2 (Score 36)
02:27 Answer 3 (Score 20)
02:53 Answer 4 (Score 4)
03:13 Thank you

--

Full question
https://superuser.com/questions/590630/s...

--

Content licensed under CC BY-SA
https://meta.stackexchange.com/help/lice...

--

Tags
#linux #commandline #sed #awk

#avk47