How Do I Find and Replace Paragraph Markers With a Space?

MacBook Pro-2011.. high Sierra … lots of memory. My problem is I have a file that has about three words per line with a paragraph mark at the end of each line. It came as an email.
I have tried a find and replace to get rid of the paragraphs and then format a proper page of text.
Nothing works. I used to be able to do this in MS Word, but I gave up window 10 years ago.
Help!
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J Vossos