Go to the control panel, and put the system in database mode. Then, open configurator. In the tool bar, click Totals, and then Clear All Totals.
As a warning, this clears ALL of the totals; sales totals, labor totals, and tms totals. You can select only to clear Sales Totals under the same menu if that is what you want.
Don't do this while the system needs to be used though because putting the system into Database modes gives you a nice System Closed message on all of your workstation.
Once you clear the totals, I highly recommend going to System -> Restaurant and setting the Auto Start Level to database. Then, turn off the system in Control Panel and restart the computer once you have all Rex X's. Once the system is back up, go back into configurator, then System->Restaurant and set Auto Start Level back to Front of House. I've found that Micros can be a bit temperamental when you clear totals and then restart straight back into Front of House. Once you've done that, and if you have the time/know how, I also recommend opening the Database utility and doing a rebuild to clean things up, but again, this can cause things to break, so unless you have time and technical knowledge, I'd just leave it alone.