Hertz (Hz) is a display setting in Windows. Your display should be set at your monitor's rated refresh rate, ie. if you have a 60 Hz monitor, it should be set to 60 Hz in Windows. This is how you do it:

https://www.hellotech.com/guide/for/how-to-change-monitor-refresh-rate-windows-10

Valheim will respect the display settings in Windows. For example, if your monitor is rated for 144 Hz, but the setting in Windows is set to 59 Hz, then in-game Graphics settings will use the resolution you select, but will keep reverting to 59 Hz even if you select higher Hz option.