I just harvested some beets from last year, they are red in the center and the rest is a yellow white color. They are huge, so are they edible?
I believe the beets are safe to eat. I happen to have some beets that have been in my fridge for about 3 months. They are also big. What I will do in the next couple of days is what I think you should do. Clean them, cut off a bit of the top, about a half inch and the same with the bottom, then put them in foil, put a bit of canola oil and salt on them, wrap them tight, put on a pan, in preheated 400 degree F oven for about one hour, check at 45 minutes, then cook as needed 15-45 minutes more. I don't know how big yours are and if you are a bit iffy then bake at 350 F or 375 F, same time. Boiling is also a way to do it but baking brings out such a sweetness in the beets it is just an unbeatable way of cooking them for me. Good Luck.
Beets last ages if stored in cool conditions; if it isn't moldy and is still hard like a rock and not squishy, it should be completely safe. If it is a gigantic beet, it's possible it might be woody, but that's a separate question from whether it is safe.
I agree with @user33210; baking beets is definitely the best way of cooking them and brings out their natural sweetness in a way that moist cooking methods cannot. I always bake them with a little bit of salt and olive oil; just like baked potatoes.