Q: how do you change the expansion tank on 2003 BMW 325xi

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
have pin hole in tank and need to replace
(1) Answer
This is not a simple process. I'd suggest a BMW specialty shop.
If you are absolutely sure you can do it, you can find great repair manuals with step-by-step instructions, diagrams, parts locations and other important information here:
Or here:
Make sure you use the OE blue antifreeze and nothing else. If there is already another coolant in the system make sure it's flushed out well.