How can withen my epoxy grout that has turned yellowish ?

You can likely use Waterproof White Out, for a quick fix, but as I doubt it would be low cost, try to use either:

A. a White Silicon,

B. Flexseal white, in a can

Either can apply over that epoxy with a cut-off end of a Q-Tip, [or a small brush if you happen to have one] so you do not get cotton fibers in the Silicon

Aquamix.com. They make grout dyes. Visit their website. I used the color smoke and did my kitchen and utility room floor. Turned out great. Was a little time consuming and tedious but it worked.

https://www.houzz.com/discussions/2287705/has-your-white-epoxy-grout-turned-yellow

https://www.wikihow.life/Whiten-Grout

https://www.ehow.com/how_6978711_clean-yellow-stained-grout.html

http://www.dslreports.com/forum/r21808175-Yellowing-grout-in-refinished-bath

https://homeguides.sfgate.com/whiten-grout-turned-brown-28968.html

Just use grout bleach.

I used Soft Scrub. It has bleach in it . I cleaned the grout on a kitchen floor with it using a bristle brush....then sealed the grout with a spray on sealer...following the directions on the bottle

Grout bleach will not work, nor will Soft Scrub. Epoxy won't bleach out. It's really easy to remove the existing grout and regrout.

you can spray it with peroxide and let it work and then just wipe it down with a clothe .You may have to use a toothbrush but unlikely

I was looking at the rest of the shower there and I would strip and regrout, it looks like that grout is coming out, it won’t take long until water gets in back there and start molding, this happens pretty quickly too, then you’ll be looking at a much bigger problem then yellowing epoxy, what I would do, after stripping and making sure u have no water damage in the joint use Mapei grout with the grout MAXimizer additive this will keep the grout from changing color and keeps it strong, as an extra measure I would seal afterwards, following instructions, with aqua mix sealera choice gold, pricey but well worth it.

Bad grouting I would say is one of the major reasons why you would be looking at water damage in between walls aside from poor tile adhesive placement.

good luck and nice tile by the way.

I have to concur with Ana...the grout is starting to come out you can tell that by the "bubble" looking pieces especially in the corner. If is a bigger job than you were looking for but if you do not do it now it is going to become a huge job meaning tearing the whole thing down because the moisture will seep through it in a heartbeat. I was going to say that if you have a steady hand you can actually paint it with a fine art brush but that is chasing good work after bad. My apologies for the bad news but it is fairly easy to cut out the grout and redo it for your piece of mind and a very expensive repair job. Good luck. Oh, if you rent then the landlord is responsible for this.

Was it grouting that was used to fill in the centre back seam? From the bumps and smears, it looks to me more like silicone chauking......

Epoxy is tough to rewhiten once it’s turned but if you want to give it the old college try make a mixture of one part water, one part vinegar, one part hydrogen peroxide, one squirt of Dawn dish liquid and 2 tablespoons baking soda in a spray bottle. Gently turn the bottle back and forth DONT SHAKE until it’s mixed and immediately spray on the grout. Let it sit for about 15 minutes and then scrub the grout with a soft brush like an old toothbrush. This solution works great on getting out every kind of stain on fabric and Formica so hopefully it will work for you. Good luck!

In case you are tempted, do not re-caulk with silicone as it will yellow too. Best long term solution is to re-grout as already recommended.