Most of the time, Windows 10 will automatically detect the optimal resolution for your monitor(s) - so there's no need to change it. However, if you want to manually adjust this or find that the auto-detection doesn't work correctly, you can do it by following these simple steps.
First, right click the desktop and select Display Settings:
Now, click the Resolution dropdown box and choose the resolution you'd like to use. Normally, Windows 10 will mark the native resolution of your monitor as "Recommended".