I want to pursue MS in mathematics abroad (especially from US, Australia or Canada). Can someone provide information about the GRE exam?

For admission to the MSc in CS course of the Uni of Madison, GRE score is not required. But the university can consider this if the score is available. A Bachelor's degree in Computer Science or a related degree is required. Also, some programming experience is required. Other application materials

Yes, you can switch between a test center and GRE Home Edition. But you must pay the standard rescheduling fee of INR 5,000.

This change must also be completed at least 4 days before your original appointment through your ETS account.

You must reschedule your GRE at least 4 days (96 hours) before your appointment.

For a Friday test, Tuesday is final deadline. 

Missing this window results in the forfeiture of your entire fee.

Yes. Many universities now offer merit-based scholarships based on your UG GPA, research papers, and internships.

Some specific fellowships still require GRE, most modern programs focus on your overall professional and academic profile.

No. Visa officers focus on your university admission letter and financial stability. If a top-tier university has waived the GRE and granted you admission, the visa office will accept that decision without requiring a separate test score.