Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Treat any new decorticate-like posture as an emergency.
Call your local emergency number now, particularly if the person is unconscious, difficult to wake, breathing abnormally, having a seizure, or has recently injured their head.
Follow instructions from the emergency call handler
Routine follow-up becomes appropriate after emergency care, when the healthcare team may arrange neurological review, rehabilitation, or monitoring for recurrence.
This is not a symptom to monitor at home or save for a routine appointment.
The article also says
While waiting for help:
Check whether the person is breathing normally
Begin CPR if they are not breathing normally and you are able
Keep the area around them clear to prevent injury
Note when the posture or change in awareness began
Do not force the arms or legs straight.
Do not restrain the person, place anything in their mouth, or give food, drink, or medicine.
If a fall or head injury may have occurred, avoid moving them unless there is immediate danger or movement is needed for CPR.
Every sign above is quoted from the sofidex article on Decorticate posture. If you are not sure, err upward — it is always reasonable to be checked.