Other forms: retrofitting; retrofitted; retrofits
When you retrofit something, you replace old parts with new ones. You might retrofit an antique car with updated brakes and seat belts, for example.
You can use retrofit as a verb, or it can be a noun meaning the new part itself, or something that's added on later. The solar panels on your two hundred year old house are a retrofit, and so is the fancy new stereo in your beat up old car. Retrofit has been around since the mid-1950s, when retroactive and refit were combined into a new word.