To improve mental fitness, use your brain power! You don’t have to do any type of exercise with your brain physically of course, but mentally you do!
For starters, always keep yourself learning. Learning new things keeps the mind active and that means health. Someone that is learning new things that interests them is likely to continue to having a mental state that’s positive and motivated.
You should also use challenges to help power you through your day and your life. Giving yourself the ability to overcome challenges from puzzles to problems in life (yours or others) can help to keep your brain working. Asking questions, getting the answers and working at it helps to improve the brain’s function, allowing you to fend off Alzheimer’s and other conditions like it.