How do I Join the line above after current line?

You can also use the ex-command

:m-2|j

• m-2 has the effect of moving the current line to 2 lines above its current position; this switches the position of the current line and the line above.
• j joins the current line and the line above, inserting a space between the two. Use j! if you don't want the space.
• | separates the 2 ex-commands

This ex-command is a short way of writing the following

:move .-2
:join