If you give a bit more of information about your budgie it would be helpful.
Is this a new budgie? If not, do you have a good bond with your budgie?
Is he currently moulting? Does he only bite when you place your hand inside the cage?
Budgies will bite hard as a negative response to express their disapproval or when feeling threatened/scared. If your budgie is new and the bonding process is in the initial stage, then your budgie is still not ready for a closer interaction and it's best to respect the safe boundaries set by him.
There are also other factors that can contribute to this biting, budgies when moulting can get more moody and this will be translated into biting when they are not receptive to having a closer interaction with us. Age can also be a factor, when budgies reach maturity they can go through more of a rebellious stage where they will bite. Hormones/breeding condition can also affect their moods.
If there is no biting when your budgie spend out of cage time with you, then it would be important to see if there is a particular toy (especially mirrored toys) your budgie is attached to as this can be causing the territorial behaviour.
Some pet birds are also very protective over their cages and can show aggression when we "invade" their territory.
It would be a good idea to rearrange the cage and switch some toys around too.
Depending on the case, you can also work on using positive reinforcement techniques to address the biting issue: