Sadly, most of us have to spend a lot of time sitting. Sitting is not a good posture to start, but probably even worse is that it encourages immobility. The key to improving the problem is to break up the time seated with brief breaks of movement. Stretching itself is less important than the movement. A standing desk is great, and a large part of the reason why is that it gets you to shift and move more than a chair. (Ergonomic chairs don't do this.) Any opportunity to walk is also very beneficial.
There are many great stretches you can do, the first priority is moving at all, then bearing your body weight on your feet. I suggest the two shown because they are so simple and are done standing. These are also dynamic stretches because they are done with movement. A stretch that you hold is called static.
Emphasize frequency of breaks over duration. The routine shown takes one minute, and even shorter with fewer repetitions would be fine. Try to do this two or three times per hour, at least.
First is the back of the leg and the hip joint. Extend one leg almost straight, pulling the toes toward you, and as you stretch the back of the leg pivot the hips to get different angles.
The second stretch involves straightening the upper spine, pulling the shoulder back, rotating the arm up, and leaning to the opposite side. This is the opposite of the hunched forward position typical of sitting at a computer.