Hi there!
I have checked your this link: https://smartsiteservices.com/clients/mundy
It seems there are some custom changes to your logo height and margin padding values there. Let me explain.
Your Header height is 80px but you have set your logo image to 90px, this causes the header container holds extra space below. Please see the screenshot:
https://prnt.sc/26kzedr
Please lower your logo image height.
You have set top margin for your logo column: https://prnt.sc/26kzdom. Please remove that margin.
There are some bottom padding for menu column: https://prnt.sc/26kzf2u. Removing that padding will finally help you get your header fixed I hope.
If you didn't set those values manually, then using the following custom code will also serve your perpose. You need to put the code snippet in your custom CSS:
#sp-logo {
margin-top: 0px !important;
}
.logo-image {
height: 80px !important;
}
#sp-menu>.sp-column {
padding-bottom: 0px !important;
}
Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Best Regards