With sed (very dangerous to put tab character like this because it's easy to accidently copy and paste, and turn tab into space):
sed -e 's/ /x/g' myfile.txt
Better with awk:
awk '{gsub("\t","x");print}' myfile.txt
sed -e 's/ /x/g' myfile.txt
awk '{gsub("\t","x");print}' myfile.txt