Happy To You
If someone says Happy Holidays to you - Thank them. Wish them the same
If someone says Merry Christmas to you - Thank them. Wish them the same
If someone wishes you Happy Chanukah, Kwanza, Diwali, Yule tide - whatever/whichever glad tiding they offer you - say Thank you. People are wishing you Happy Holidays or Merry Christmas for the joy of sharing their hope of a peace filled season of light with you.
You are free to wish others the greeting of your choosing, there is no ban on saying Merry Christmas. Someone saying Happy Holidays is not an attack on Christmas, long before television and nightly news programs people were saying Happy Holidays as a greeting.
Not all people are Christian; not even those that celebrate with a Christmas Tree and Santa. Not every person living in the U.S.A. is a descendent of those that populated Plymouth Plantation and there are cultural festivals leading up to Christmas for many non Anglo-Saxon cultures.
This should be a joy filled time of year for reflection and well wishes. Offer your wish with joy and love and accept the Happy Holiday or the Merry Christmas or the Happy New Year with joy and gratitude.