If you have submitted the application form but have not yet paid the fee, you can edit the form. Because the application fee has not been paid, the application is likely considered  as incomplete, allowing you to edit it before final submission. Here is exactly what you need to do to edit History for the General Aptitude Test (GAT) for your BHU application:

Edit Before Payment (If Registration is Open): If the registration deadline has not closed, most entrance exams like CUET allow you to log in back in and modify your subject choices before clicking on the final "Pay Fee" button.

Application Correction Window(If already submitted): If you have already submitted the form, you will not be able to edit details in the application form. Please wait for the correction window. During this time, the NTA allows students to change Subject/Test Paper codes.