I do not understand what you are trying to do.
In your example, you first set a variable equal to "alok 10". Your SED code is perfectly valid, and will give you: "alok 10 10"
Note: don't use the -i flag unless you are operating on a file and want to edit it "in place". You also do not need the -e flag.
echo "alok 10" | sed 's/10/20/'