Welcome to healthcaremagic forum.
Since you had the habit of smoking
,the brown spot on roof of mouth can be of stomatitis nicotina
type of lesion caused due to the habit of smoking.
I would advice you to discontinue smoking.
Take plenty of water,fruits as well as nutritious diet.
Vitamin -A as well as folic acid
supplements has to be administered.Chemotherapy
or excision of lesion has to be done depending on the severity.
Meanwhile,maintain oral care.
Hope this helps.