How can I reduce the severity of dementia?

Functioning changes. The severity of dementia cannot be changed currently but by changing a person's environment you can help a person be more functional. In early illness some dementia specific medications can slow the loss of functioning.
Use your brain. Dementia is a broad category of illnesses that have cognitive and memory impairment as the main symptoms. Alzheimer's is the best known and most common form of dementia. There is some evidence for the value of continuing to challenge your brain with mentally stimulating exercises and work for as long as you can. Also check info from alzheimers association website.