This is the simple answer, every three days, clean your vase, put in fresh water and trim at least one inch off the base of the stems of the flowers and put them back in.
At Let me knot we strive to provide fresh and quality flowers, however, proper care ensures that your flowers last the longest they can. Bacteria buildup is what is killing your flowers off. So please be sure to change their water every three days at minimum. Go the full length of trimming their stems and sterilizing your vase and you will get the most out of your flowers.