1. (noun) a component or accessory added to something after it has been manufactured

• component
• constituent
• element

2. (noun) the act of adding a component or accessory to something that did not have it when it was manufactured

• addition

3. (verb) provide with parts, devices, or equipment not available or in use at the time of the original manufacture

• furnish
• provide
• render
• supply

4. (verb) fit in or on an existing structure, such as an older house

• instal
• install
• put in
• set up

5. (verb) substitute new or modernized parts or equipment for older ones

• modernise
• modernize
• overhaul