From the navigation tree, double-click VLAN.
From the VLAN tree, double-click Spanning Tree.
From the Spanning Tree folder, click Globals.
In the SpanningTreeAdminMode field, select the STP mode.
Click Apply.
Click Yes to the warning message reminding you that you must reset the switch for the change to take effect.
Click Close.
To reset the switch, choose Edit > Chassis.
From the System tab, choose the reboot option and click Apply.