update issue and currency switch

Hi,
1. In my wordpress plugin dashboard is showed:
Automatic update is unavailable for this plugin.
I downloaded the new version and tried to install over the old one. But installation failed.
How to update the plugin correctly? And where can I see what’s new in the updating version?
2. I set a default currency USD for the whole website and a checkout currency as RUB only as I can accept only rubles.
While checkout USD switches to RUB automatically, but if a user abandons the cart, overall website currency switches to RUB, so a user needs to select USD in the currency bar. How to fix it?