Very simple to install. About the easiest repair job on a Tivo. You will need to open the Tivo up, which itself requires a torx driver(a screw driver for sort of star shaped screws). There are online guides to be found detailing the task, if you need help(recommended).
You can get the official replacement fan from weaknees.com for about $20 or so(total shipped price, last I checked).
You can find Tivo replacement fans on ebay for about $10 shipped, which people seem satisfied with.
You can also use appropriately sized computer case cooling fans from places like newegg, etc. If you go that route, make sure it is deemed(labeled, etc) to be a "quiet" or "silent" fan. Some computer fans can be very noisy.
If you do some searching here, you will be able to find threads on the topic for more details.
Tivo HD Lifetime 1TB int, 1TB ext
Tivo 540040 400 GB Lifetime
Tivo S2DT 750 GB msd
Toshiba sdh400 r.i.p.