Replace the whole body. All the cancerous, pre-cancerous, and worn down, aging and diseased cells wholesale.
Grow monoclonal headless human bodies in a lab. Innervate, artificially supply with hormones, and grow until puberty. No head. (Genetically or surgically stop precursor cells early on in development.) Grow the bodies artificially in pig uteruses at scale. Make it a monoclonal line that is O-, HLA neutral, etc. to avoid transplant rejection and the need for immunosuppressants.
Before these are ready for human use, they can be used for population studies and in situ experiments. They can also be used for organ transplants.
Develop and perfect human head transplantation. Try it on terminal patients, paralyzed patients, et al. Get good at it. Early results won't be good, but given time to develop, we might be able to reattach the spinal system.
Transplant recipients get a new pulmonary, cardiovascular, and circulatory systems. New thymuses with better immune systems. This could be a non-negligible boost to human lifespan.
New bodies can be genetically modified to have higher VO2 max and other stats. You could choose the height, build, and gender of your new body. Literally everything.
This is better than duct taping aging components in place. It's replacing everything but the head and brain, which is where we should be moving as a society anyway.
It won't cure brain, blood cancers, or Alzheimer's. But it'll be useful against heart disease, lung disease, liver disease. Everything else.