Can I reduce my risk of getting dementia?

Probably. Don't become obese or develop type 2 diabetes, control your blood pressure & lipids, stay physically active & exercise & keep your brain active w/ work or hobbies, stay active socially. In other words, try to lead a healthy, happy life. Also, choose your parents wisely, there's a major hereditary component to alzheimer's.
Probably. Right now, the only fairly good evidence for reducing risk of dementia is exercise for an otherwise healthy person. There is evidence that if you have health problems lke high blood pressure and diabetes and vigorously control them that this will reduce risk.