A chicken’s beak is really important to their overall health. They obviously need a strong healthy beak to eat, but they also use their beaks as we would use our hands, so exploring, cleaning their feathers, nesting, making dust baths are among the many uses a chicken has for their beak.
A chicken’s upper and lower beak should almost the same length, with the upper beak just slightly longer. Chickens with access to pasture will keep their own beaks trimmed and sharp. Chickens kept in unnatural environments may need to have their upper beaks trimmed if they cannot file them down themselves. This can be done with nail clippers or dog toe nail clippers. Be careful not to cut any live tissue. Caged chickens with not enough space are often times completely debeaked so they will not harm their cellmates. Factory farming in this way is not healthy for the chicken or the farmer, and certainly not the consumer of the eggs or meat.
A crossed beak is a genetic abnormality that will be noticeable from birth. If you get a crossed beak bird do not breed them, as they will pass that trait on. Their beak can be trimmed, but it will grow back to its deformed state. If you have a chicken with a crossed beak, the chicken will have some issues, but they can usually live a decent life. My neighbor actually has one with a crossed beak. I watched the chicken today eat her feed, and she seemed to eat fine, so hopefully she will be OK.