sed 's/:/\n/g' filename
This should do what you want.
sed --- calls the sed program.
' --- wrap "code" in single quotes.
s --- substitute command
: --- any ":" matches.
\n --- replace matching ":" with "\n" which is a "new-line" character.
g --- do the command globally; that is, at every occurrence on line.
" filename" ---space then filename.
If this works for you as evidenced by what rolls across your screen then simply redirect your output from the screen to a file like this:
sed 's/:/\n/g' filename > new_filename
choose a new filename or you will clobber the old one and end up with a zero-byte file or use the -i option, as mentioned above.