for f in * ; do mv "$f" $( echo $f | tr ' ' '-' ) ; done

rename old_value replacement_value directory/filename

Rename all files which contain the sub-string 'foo', replacing it with 'bar'

for i in ./*foo*;do mv -- "$i" "${i//foo/bar}";done

rename 's/ /-/g' *

As long as you have perl based rename.

You can check:

rename --help

or

rename [-v] [-n] [-f] perlexpr [filenames]