Instructions on how to change your major in Workday can be found here: https://hub.wpi.edu/article/735/workday-student-program-of-study
Occasionally, this process doesn't work to change your primary major (particularly if you're a double major). You or your academic advisor can get it fixed by emailing registrar@wpi.edu
To maintain "satisfactory academic standing" full-time students need to pass (not just take) 4/3 units per semester. Typically this is 4 normal undergrad courses, but the math gets weird with things like PQP and PE courses that count for 1/6 unit and 1/12th unit respectively
That policy is documented in full detail here: https://www.wpi.edu/offices/registrar/policies-procedures/academic-standing
IQP counts for 3/3, usually in the same term if at a project center, and most often spread across 3 terms if on-campus (though the distribution can vary).