hello, you can apply baking soda over it or do baking soda in water mouthrinses. this will help to heal your ulcer. if they still persist, you need to consult a physician and get your vitamin B levels checked. if required take supplements for it.

If u have painful sore mouth, do gargle or rinse with Benzydamine oral rinse 15 ml solution thrice daily. Avoid spicy and hot food and drinks. Apply Mucopain gel locally on the sores with clean finger tip 3- 4 times daily especially before meals. It will make the areas numb so that it won't hurt u while eating. Make sure u have a clean stomach, no constipation or liver disorders. Take a vitamin B-complex capsule daily after dinner  for a month since these ulcers occur sometimes due to vitamin B-complex deficiency. Drink lots of cold water and maintain a good oral hygiene. The ulcers will naturally heal within few days.