I can't responsibly suggest exercises without evaluating a patient. It's best to see a PT for this. However, generally speaking, if you've been in one position for a long time, you might want to try stretching in the opposite position. You also should set yourself an alarm every hour or so to remind you to move around.

There are quite a few but here are 2 of my favorites.
Pelvic tilts - sitting with feet on the floor and shoulders upright, tighten the stomach and flatten the back, hold for 5-10 count
Upper trunk rotations - sitting with feet on floor and shoulders upright, rotate the shoulders and arms to the left then to the right, repeat 5-10 reps.
Of course these are very general and to get specific exercises to address your issue, consult your local PT. Good luck.

If you are able to change positions every 30-60 min, that would likely be most helpful. Marching your feet under your desk can keep your core active and may relieve some pain. I recommend making an appointment with a physical therapist to better assess your needs.

There's no such thing as perfect exercises or perfect posture. However, pinching your shoulders repeatedly, getting up to go to the bathroom/grab a coffee, or just changing positions frequently can be helpful. Also, make sure you are active outside of work with exercises/strength training/cardiovascular training and an active lifestyle can be very helpful.