how to "find and replace" a manual line break with a paragraph line break?

Does anyone know how to “find and replace” a manual line break with a paragraph mark?

image description

See \n description in help.

I did and it didn’t work

Then you did that wrong. But unfortunately you don’t tell what you did, e.g. what did you put into “Find” and “Replace”, and which options did you use in Find & Replace dialog.