It’s due to excessive heat in your body and faulty diet habits. You need to give more details to remove this problem from the root cause. For now you can apply smile mouth ulcer gel on that. For medicines you can consult online

sootshekhar ras 1 tablet twice a day chandanadi avleh 10 gm twice a day

Find out root causes by ayurvediy way consulting privetly. Apply honey  on affected parts. Improve your diet and digestion process to overcome this problem

Your problem is due to fungal infection , Apply Tankan Bhasma mixed with honey , or Place 2 tsf IRIMEDADI OIL in mouth for 5 minutes and then throw out thrice a day  also take Tab Septelin Himalaya 1 bid  Brush four times in the day, Mouth wash with Babul leaves/Neem leaves/ Chameli leaves with flowers  boiled and cooled water, Take Arjun Churna or quath bid, Triphala Churna 1 tsf at night with Lukewarm water for 3 days