I don't like to label them mean , because then our brain thinks that , and makes for negative energy.
How long have you had ?
If they don't take treats nice , I start with a treat dish , or setting treat down for them to pick up . So if it's a set treat down , at first after I set it down I move my hand away , I say good bird when treat is picked up . After all get the hang of this . Then when I set the treat down on a flat surface, I move my hand however far enough away they are non reactive to to it, and eave my hand in the treat position thumb and pointer touching flat surface and not moving. After that goes well fir several times , I move my hand less far away. I always pair good birdie when they pick up treat . Until eventually I leave my hand right next to treat , and then I start trying to hand treats again when all of this is non reactive.
Shaping, doing things in little segments until you get to the desired result or behavior is very successful way . It's how zoo keepers train wild Creatures. It's fluid to you and you adjust to the individual and how it's going..have several positives before you adjust to the next step , often several times over several days before you move to next step ..
It would be the same with a treat dish. At first you would say hi " name" drop treat in dish back up . Good birdies when they come and take it. Then you would move to hi name come here , show treat and they at least have to look or move a step towards dish before you drop it and back up. Working up to they have to come all the way to the dish before you drop it and you stay there. Then working to being able to hand it to them .
Bites , read body language, try not to put them in the position they have to bite before you listen. If bite happens, no bite, if pressure but not true bite say be careful. If bite happens , no bite, then I just just shift them , to chair arm , to my leg . I don't hold grudge, I don't make it dramatic, it's the same thing every time. If 2 bites happen in shirt order then back to cage for five minutes, keep it short then let them back out right away again.