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:
Then scroll down and click Advanced 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", if you want to restore default settings at a later date. Once you've chosen the resolution, click Apply: