This would mean changing the slot which is only possible when the bootloader is unlocked, however its worse a try. So find out which is the active slot and then via fastboot change the slot
Reg. going back to previous version by changing slots, here a discussion
Overall, before changing slots etc I would have a look if for some reason the faulty GPlayServices update also causes issues when LOS is used, i.e. deinstall the GPlayServies updates in the App. More information in the topic linked above by TobiasF or does is not boot at all into system?