Move in to a house that the tile in the kitchen , bathroom and entry way have been mopped but grout never cleaned. The original color of grout is a light beige and it is now a dark brown to black

You can try some grout cleaners but results may not be acceptable. Replacing the grout (with the correct tools and care, not as difficult as it sounds) may be a better option.

I wouldn't replace the grout unless it was too low or had some other defect. First try cleaning with a heavy duty acidic tile and grout cleaner, there are many. If that looks good to you, apply a few coats of grout sealer and that should be it for several years.

If not, get some grout colorant and apply per directions. Aqua-Mix is a brand that I've used and vouch for. Grout stains/colororants can take a long time to apply, but much less work than replacing. It's a water base epoxy, so you shouldn't have to seal again.

https://www.groutcolorants.com/?leadsource=PS104&s_kwcid=TC|6221|aqua%20mix%20grout%20stain||S|e|24 956837779&mkwid=sKdeR0g9Y_dc|pcrid|24956837779|pkw|aqua%20mi x%20grout%20stain|pmt|e|&gclid=CIm7vM-YlboCFWJp7AodjnQArQ

Jaz