My father has dementia and he gets angry easily when I ask him to do something. How do I cope?

Make an appointment. With a geriatric psychiatrist who can help him.
Redirect. Behavioral problems in dementia are very common. Irritability is one of the most common. First, don't argue with the dementia(it will always win). Try redirection if one can't past the trigger. At times we need to just let it go and move on. Hoever if the irritability has an impact on care or safety sometime meds are needed.