From the recent report of the National Academy of Sciences, Engineering and Medicine come general recommendations.
Cognitive Training: there is currently no evidence to support a conclusion that cognitive training can prevent or delay mild cognitive impairment or Alzheimer's.
Hypertension: manage blood pressure in those with hypertension (particularly in mid-life i.e.. 35-65) to prevent, delay or slow the progression of Alzheimer's.
Physical activity: studies fail to demonstrate increasing physical activity prevents, delays or slows mild cognitive impairment or Alzheimer's. However, the report noted the other health benefits of increased physical activity such as lowering the risk of hypertension, stroke and obesity as well as symptoms of depression.
Contrastly,to improve your life overall: consider the FINGER study out of Finland which found that targeting diet, exercise, vascular risk factors and brain training slowed cognitive decline in older adults.
Downloadable report: nationalacademies.org/Dementia