you can even do square edge tiles, just take your time and do it gently
anyways
hand tool, cheap as hell, just take your time, its hard work though, you just rake it out, its easy to slip and mark the tile as it really does take some strength, so a electric one is much easier
https://www.screwfix.com/p/pro-grout-rake-cutter/17903
electric one, 1 million times easier, take it slow and its easy enough
https://www.screwfix.com/p/vitrex-gr...-remover/75540
you will then need to choose some grout, whatever colour you want
a spreader, just work the grout into the gaps
https://www.screwfix.com/p/vitrex-gr...questid=790272
a sponge to clean up
https://www.screwfix.com/p/vitrex-gr...-x-130mm/1127f
thats it, just watch a few youtube videos and you will see its easy enough to do, it really is a DIY / handyman job