The best way to regrout shower?

My shower was built in the 70 and need to regrouted it is come out in some places and real moldy in other places. I can not afford to have some one come in and do it. I have seen tools that takes the old grout out was woundering if they work and and how hard it would be. I have moved in to take care of my very ill mother. So there is lots of thing that need work here. Thank you

Some of the tools work but being an old shower it will be tough. Utility knifes work also but be careful

I would get rid of the mold first with bleach. Since you are removing the grout the bleach will be fine.

What is it about the grout that says it must be removed?