High Blood Pressure can have many causes beside Misaligned vertebrae. Often dehydration is a co-factor as well and an imbalance of the calcium - magnesium household in the body. When I check the spine of people with High Blood Pressure I often find misalignments in C1 (Atlas) and C2 (Axis) as well as Th2, Th12. If the patient is easy becoming ´pressurized´, I mean angry, then often an additional misalignment can be found on Th4 and Th5 (Gallbladder, Liver connection.
There is a self therapy exercise featured on the Dorn Method website for the neck however I recommend to do all basic 4 self therapy exercises because if one treats the whole spine then the chances of improvemnt are higher.
If the misalignment of the atlas can be felt (palpated) then you can do a specific self therapy exercise on that area first but you should also do the other side.
To do that just locate the Mastoid Process behing the ear and apply gentle counter-pressure using the thumb directed inwards and slightly upwards while moving the head relaxed and reasonable fast from side to side (a no no movement)
Blessings