Hi John: Here is site that tells how to do that and gives a recipe so you can have a sparkling clean shower: https://experthometips.com/how-to-clean-grout
Baking soda and vinegar and scrub with a toothbrush brush.
or you can use the CIF mold remover which works just as well.
pls read the instructions before use as contains bleach.
I personally use Barkeeper's Friend powder - make a paste with water-and a Scotch Bright pad. Or a firm/hard toothbrush (dollar store used just for cleaning).
And nitrile gloves, of course, to protect hands and manicure.