What does this mean?

I was reading financial times and a sentence was:

Singapore Airlines is exploring options to sell all or part of its 49 per cent stake in Virgin Atlantic to..............

what does stake mean here? equivalent of stock?

and this as well

An upgrade from HSBC helped Swedish truckmaker Volvo buck a sharply lower European market

what does "An upgrade" and "buck" mean here?

I suspect the upgrade in question here refers to a credit rating given by some sort of oversight board; a better credit rating would make a company look far more appealing to investors.

It could also be that HSBC (apparently a big Chinese/British bank), gives stock ratings. Perhaps Volvo stock was upgraded from "sell" to "hold", "hold" to "buy", "dump" to "sell", "underweight" to "overweight", etc.

I suspect, on the day/week/etc. in question, the European stock averages were sharply lower, but Volvo stock went up because of the upgrade.