SerialConnection.Reset
From Xojo Documentation
Method
Resets the connection's baud and byte format.
Notes
The serial connection must be open for this method to function.
Changes to the serial device are queued so that you can modify multiple properties of it without them all taking effect immediately. For instance, you can set the baud, parity and stop bits as one operation instead of three. The same thing applies to the line state properties. Once you've made all your changes, you need to call .Reset on the serial device to reset the device to the new settings.
Example
SerialConnection1.Reset