How the fungal infection of mouth shoul be treated?

Are you sure it is a fungal infection?

Treatment of Oral thrush

The goal of treatment is to control the growth of Candida. Antifungal medications (such as nystatin, clotrimazole, and miconazole) are given for 5 to 10 days. They are administered in the mouth (swallowing the medicine after "swishing" it around).

A single-dose oral medicine called Diflucan is available if these treatments do not work. Underlying conditions/disorders should be identified and treated.

https://www.drugs.com/cg/oral-candidiasis.html

I would say see a Doctor.