Like this:
# No backslash before space needed, as the entire sting is quoted with "
filesDirName="/C/Users/OM SAI RAM/HelloWorldSagar"
# echo allows multiple parameter, so it worked by chance
# You should quote it anyway
echo "${filesDirName}"
echo "cd ${filesDirName}"
# quote the parameter
cd "${filesDirName}"
Rule of thumb: If in doubt, quote the variable.
BTW: Indent your code by 4 spaces, so it is displayed as a code block