In the web interface go to Settings at the bottom left, then select the System Editor tab.

Click on the config.g file and an editing box will pop up.

Find the area for choosing an arm length as shown in Pic 1.

There are 2 lines in the configuration, one for carbon fiber arms, one for injection molded arms.

The ACTIVE line is the one WITHOUT the semi-colon. The line with the semi-colon is NOT active. If your configuration is correct, leave it as is.

If the line for the arms you have does have the semi-colon at the beginning, you will remove the semi-colon, and add a semi-colon to the beginning of the line that is NOT for your arm configuration.

Click save at the bottom. The web interface should ask you if you would like to reboot. Choose yes. If it does not ask you after saving, shut the machine off for 30 seconds and turn it back on to load the correct settings.