Must I fast or feed for my mother who passed away and owed two months of Ramadan fasting that she was unable to make up? And how should the feeding be?

It is best that you fast on behalf of your mother, for the Prophet, peace and blessings be upon him, said: "Whoever dies and has fasts outstanding, his guardian should fast on his behalf." If it is not possible for you or other relatives to fast, then feed a poor person for each day from her estate or your own money. The amount for that is half a sa' of the staple food of the land, and it is permissible to give all of it to one poor person.