I already written a guide for other sites, and I just added a guide for fixit.
It is a medium difficulty repair, due to the fact that there is soldering/ desoldering involved with a battery.
PROBLEM: The Dreamcast has an internal clock which is what drains the battery. When the battery is drained it doesnt keep the time, which is why it asks you to set the time and date on every boot up. SOLUTION: The battary is rechargible duh? Well why replace it, to recharge the battery turn on dreamcast set the date time etc. Now keep the system running on the Browser for at least 2 hours, the battery wont charge when power is off. Leaving the A/C cord in the system while not in use doesnt count. Now after 2 hours turn off console then switch back on, it should still be set. The internal battery will last 1 week without charge, so repeat every few days. You can still play dreamcast while charging just don't turn off power when switching games. Hope i could help, if any more issues look at user manual which can be found online.