You need to wait for the spot round (in-person counseling) at Thapar University for that. You can’t change your branch preference in the first 4 rounds (which are all online rounds). But during the spot round, you’ll get a fresh paper to fill in your desired branches in the order of your preference. This is when you can reconsider your choices. If during the online counseling rounds you feel satisfied with the branch you are getting, you can freeze your seat. This won’t hamper your chances in the spot round, but it will provide you with a backup in case your desired branch(es) runs out of vacant seats by then.
You should not worry as you will get a chance to get your desired branch at the time of In-person Counseling.
When you attend the In-person counseling, you will be allotted a fresh paper to fill up your choices, and then you can change as per your choice or interest. Till that time, you will have to retain your allotted seat.
You should relax as you have been allotted a branch at Thapar University. You can change your branch in later counseling. It is important to have patience and be updated with the counseling dates.