How to teach to an adult person that is not used to study?
Life is dependent many times on being motivated to succeed. There is not a magical formula for having no motivation and yet having success. If your mother cannot find the motivation to learn, it is rather hard to force learning down her throat. If you can make learning matter to her, she will want to learn. 

Perhaps you should look into hiring a professional tutor if your mother needs to learn this material for her employment (or the like). Removing the family element could allow for her to feel more motivation. In certain countries, there are programs for adult learning. One can obtain an equivalent to basic high school education from these programs.