How to replace newline in Excel for Mac
Solution for some cases could be:
Copy cell(s) (⌘ + C), paste special (⌃ + ⌘ + V), select 'Values', click 'OK'
Caveats: there must be space at the end of line, otherwise there will be no separation between lines (it removes newline but doesn't replace); it doesn't work in-cell (source and destination cells must be different)