sed -e 's/ \+/;/g' File1 > File2
$ gawk 'BEGIN{FS=" +"}{$1=$1}1' OFS=";" filehello;worldfoo;bara;word with a space$ awk '{gsub(/ +/,";")}1' filehello;worldfoo;bara;word with a space
Try:
sed -e 's/ */;/g' file