Hi Fisher, you will need to carefully remove the grout that's there first. After that you can re grout the tile: https://www.thespruce.com/how-to-grout-ceramic-wall-tile-1824821
Hi Fischer, depending on how bad the old grout is you may be able to use a grout pen to fix it. This tutorial will show you how - https://www.realsimple.com/home-organizing/home-improvement/grout-pen-bathroom-transformation
Here's a video for "beginners" that will show you what tools you need and exactly how to do your project. Wishing you the best.
Once you do re-grout, take the time to seal it once it has time to cure with several coats of grout sealer. It is a painstaking task but WELL worth the effort. Paint it on with a small brush and wipe gently off of tile.
Grout cleaning and grout removal and replace it
Here’s a Hometalk link for you to consider. Hope this helps