If tooth decay has gotten too severe then there's no way to reverse that tooth decay other than going to a dentist and if you have cavities the dentist will need to fill those cavities in.
You can however prevent tooth decay that has just begun but using mouthwash and gargling with it twice per day.
Also be sure to brush your teeth everyday after every meal and use some demic toothpaste which can help fill in shallow cavities by bonding to the teeth.
Oil pulling with Coconut Oil is another option for reversing and preventing tooth decay.
Take a tablespoon of Coconut Oil and put it into your mouth and swish it around in your mouth and make sure you pull the oil through your teeth and do this for around 5 to 10 minutes.
Then spit it out and brush your teeth right after oil pulling.
The coconut oil will get rid of bacteria in your mouth and prevent tooth decay, gum disease and other teeth problems.
You can also use Apple Cider Vinegar to gargle with which kills bacteria that can cause tooth decay and all you do is swish the Apple Cider Vinegar around in your mouth just like any other type of mouth wash.