First upgrades should be anything needed for a proper fit and comfort. Odds are good that if you bought the correct size bike and do not have unusual proportions, you don’t need to change out any components. But get the bike fit.

As for performance upgrades, good low rolling resistance tires. The difference between good and the best can be equal to 10 W of power or in the range of 5% of total power. The difference between bad tires and the best can be much more.

If you bought an inexpensive bike, than you may get smoother shifting with better components.

Depending on what you have, better wheels. If nothing else you should be able to get lighter wheels. While weight isn’t really all that important in terms of actual performance, unless you’re competing, it can affect how the bike feels.