what is the difference of these 2 sentences?
 

首相のかわりに、外相がアメリカを訪問した
首相にかわり、外相がアメリカを訪問した
what is the difference of these 2 sentences?

I don't think there's any difference. Maybe as much difference as there is between these two English sentences:
-Instead of the Prime Minsiter, the Foreign Minister visited America.
-The Foreign Minister replaced the Prime Minister in visiting America.

As far as meaning, no difference.

The only difference is in the level of formality. The second sentence is more formal than the first. You won't see the first in the newspaper or hear it in the news.

got that, thanks alot