In general you have to complete your BA History course after which you can move for MA History. But this can be a time taking. You can approach Distance education systems like IGNOU which usually offers courses which is open to all graduates. That is you can do MA in History from IGNOU without wasting a year. Now if you want to go for BA then its another thing. But the point is you have to pursue a course, you cant just change your degree from Bsc Maths to BA History.

I hope my answer helps. All the very best for your future endeavors!