Caulk will not last or do any good and to regrout all the existing grout needs to come out.

well, we bought a grout cleaner but it didn't do anything. So the best thing to do is call a professional? or re-grouting is something we can do ourselves?

"or re-grouting is something we can do ourselves?"

Sure, but only after all the existing grout is ground out.

Do not grout. silicone caulk only

Clarification: Use grout between the tiles, but caulk where the wall meets the floor.
I've scraped grout out with a metal grout removal hand tool; it wasn't difficult, just time consuming. Regrouting isn't difficult either, but I've read that epoxy grout is more difficult to work with. However, with all the info on DIY available, I'd certainly educate myself and DIY. I guess the trick is to do small areas and clean as you go, as it dries faster than regular grout. But it is the BEST for not staining and ease of cleaning. I have it in a 10 year old shower and have never had to scrub the grout. Just normal cleaning of the shower and tile keeps the grout looking like new.

I scrapped out the grout on my kitchen floor. They have a special tool to do it. It isn't difficult except it makes a dusty mess (wear a mask). Its just plain messy!

Jane

bought a grout saw and it seems to work really well scraping the top layer. Will see if it works it just requires a lot of elbow grease

All needs to come out, out you cannot effectively put grout on top of grout.