I had a mri and was diagnosed with white matter changes to my left external capsule(not sure where that is) is there a way to slow this disease down?

May be normal. : White matter changes are a normal part of aging. Speak to your physician of you are concerned.

White matter changes: Without understanding what prompted the ordering of the MRI, not the specific aspects and findings of the MRI (location, sequence etc etc) it is difficult to determine what these findings on their own mean. Since you have clearly had a previous health care provider interaction, suggest you speak to that provider to further discuss the relevance of these findings.