Command
:%s/<Ctrl-V><Ctrl-M>/\r/g
Where <Ctrl-V><Ctrl-M>
means type Ctrl+V then Ctrl+M.
Explanation
:%s
substitute, % = all lines
<Ctrl-V><Ctrl-M>
^M characters (the Ctrl-V is a Vim way of writing the Ctrl ^ character and Ctrl-M writes the M after the regular expression, resulting to ^M special character)
/\r/
with new line (\r
)
g
And do it globally (not just the first occurrence on the line).