Firmware upgrade question

If I upgrade the firmware on my APM2, will it reset all settings to default or can ı just carry on flying?

Upgrade will not erase your settings, you can fly.
But read release notes.

Some upgrades include new parameters that you need to be aware of and may need to tune.

It will keep your mission and old settings when you upgrade. That said you should always save all the parameters using the button in the full parameter list tab before you upgrade. I always save a new copy before I make any parameter change so I can easily go back.