There are several bolts on the bottom holding the speedo/tach to the mounting bracket. Remove the speedo and tach cables. If the cables are not tight you can unscrew the large nuts with finger tips, often you will need some small pliers to get the large nuts loose. Careful not to scratch the large nuts with pliers. Remove small Phillips head screw holding big black plug for wiring harness on to back of speedo/tach.
Once you have the piece off the bike and in your hands it's disassembly will be self evident. A number of small screws, be careful with them these parts are no longer available from BMW and the small screws are not a size you will find in the corner hardware store.
Parts #11, 4 and 16 are the seals that keep moisture out. It should probably be considered moisture resistant and not moisture proof under the best circumstances. In addition the glass lenses on the front are often not sealed too well. After you dry it out and air it out it will be fine till the next time you ride in the rain maybe or if it gets very humid the moisture can accumulate on it's own. We used to replace these seals sometimes but they are not available anymore. You can't just glue the whole thing together because the unit does need to be able to be taken apart for repair.
Look for cracks in the plastic case pieces especially part #2. These you might repair with some RTV or something.
When you put it all back together do not over tighten the screws. This is how the covers often get ruined. No more than snug is tight enough.