Moulting happens 1-3 times a year depending on the species of bird you have. You may notice your bird have some personality changes like biting and screaming more than usual. Birds will generally show a change in appetite, often eating less despite the fact that they use up to 30% more energy when moulting. This is why proper nutrition is important and birds may experience various vitamin deficiencies during this time. If your bird is not eating enough balanced meals (good pellets, vegetables and fruit) you may consider adding vitamin supplements to aid them (consult your vet first to ensure you don't overdo any vitamins or minerals).
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Have patience and understanding during this time, your bird is going through a rough time on their body and they will return to their normal selves. Give them space when they need it and love when they need it. Moulting and growing new feathers leaves your bird itchy and uncomfortable so provide lots of bathing opportunities (play around with temperature and how water is presented to find what your bird prefers). Your bird may appreciate you giving had scratches and gently helping with the pin feathers on their head. Remember pin feathers are very sensitive and while developing have a blood supply. If a forming father, also called blood feathers, is injured make sure the whole feather to the root comes out to properly close the pore and stop the bleeding. If bleeding doesn't stop you need to see a vet, birds don't have a lot of blood so any blood loss can quickly turn life threatening.
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Irritability and boredom may lead to birds overpreening and even plucking during this time. To reduce this risk distract your bird with lots of preening toys (toys that encourage chewing and preening) and plenty of foraging.