Question

how could the incas go to the aztec

Answer 1

By walking or riding on horses....lol

Answer 2

It is currently assumed that the two powers never had direct contact with one another. The geography separating them was quite difficult to traverse without advanced transportation, although it is likely limited trade occurred through the use of intermediaries. Little is really known for sure, of course. Obviously a direct land route over smooth terrain was out of the question. Where Central and South America join would have been quite complicated to traverse. So walking is out, and horses would have been difficult to acquire seeing as how they had been extinct in the Americas for thousands of years until the Europeans arrived with their own horses.