Question

how does tooth decay can be treated

Answer 1

it can be treated either by root canal or doing the use of hydrozen peroxide

Answer 2

For early stage tooth decay – your dentist will talk to you about the amount of sugar in your diet and the times you eat. They may apply a fluoride gel, varnish or paste to the area. Fluoride helps to protect teeth by strengthening the enamel, making teeth more resistant to the acids from plaque that can cause tooth decay.
Your dentist may discuss a filling or crown with you – this involves removing the dental decay, offering local anaesthetic to numb the tooth and filling the hole
If tooth decay has spread to the pulp (in the centre of the tooth, containing blood and nerves) – this may be removed in a process known as root canal treatment.
If the tooth is so badly damaged that it can't be restored – it may need to be removed. Your dentist may be able to replace the tooth with a partial denture, bridge or implant.