How can I keep a bouquet of roses fresh?

Here are options for you.

https://m.wikihow.com/Keep-Roses-Fresh

https://www.ftd.com/blog/create/mythbusters-keep-flowers-fresh-longer

https://www.proflowers.com/blog/how-to-make-flowers-last-longer

Roses normally last about 4 to 5 days. Keep the water clean and you might be able to stretch that.

cutting the ends off a little , rain water, add a little sugar, used water that has been sitting out for a day or two so clorine will dissapate

Everyday you should trim each stem on the diagonal under cold running water before putting them back into the vase. Pour some clear soda into the vase along with a Tbsp of bleach to kill bacteria each time you change the water.

Pour lemonade in the container this will extend the life of the roses. Dave