I think this should do the trick in a background script or such
var taskSLA = new TaskSLA(/* sys_id or task_sla GlideRecord */ task_sla);
taskSLA.setUpdateTime(/* glide_date_time, GlideDateTime or string "Value" */ end_time); //Optional if you want to set the end_time to calculate from
taskSLA.updateState(TaskSLA.STATE_COMPLETED); // or TaskSLA.STATE_CANCELLED
