Explanation

From the second command output (show spanning-tree mst) we learn that MST1 includes VLANs 10 &
20.Therefore if we want DSW1 to become root bridge for these VLANs we need to set the MST 1 region to root ->The command "spanning-tree mst 1 root primary" can do the trick. In fact, this command runs a macro and setsthe priority lower than the current root.
Also we can see the current root bridge for these VLANs has the priority of 32769 (default value + sysid) so wecan set the priority of DSW1 to a specific lower value. But notice that the priority must be a multiple of
4096.

Reference:
SSO HSRP alters the behavior of HSRP when a device with redundant Route Processors (RPs) is configured for stateful switchover (SSO) redundancy mode. When an RP is active and the other RP is standby, SSO enables the standby RP to take over if the active RP fails.
The SSO HSRP feature enables the Cisco IOS HSRP subsystem software to detect that a standby RP is installed and the system is configured in SSO redundancy mode. Further, if the active RP fails, no change occurs to the HSRP group itself and traffic continues to be forwarded through the current active gateway device.