Thank you for your question.
You can try achieving this using Header 6 or Header 7 header layout in the theme header settings which displays hamburger menu style as listed below.
Okay thank you. But is it then possible to have the menu in a popup style covering the main content rather than sliding it to the left?
On this note, is there a way to modify the preset headers to have the menu (hamburger stalye) and the contact details in one line?
To achieve this, you have to develop custom code in the child theme of Jevelin theme.
Developing custom code for custom functionality is beyond the scope of support that we provide here. Please see https://themeforest.net/item/jevelin-multipurpose-premium-responsive-wordpress-theme/14728833/support
If you are not a developer then you can consider hiring a developer to develop it for you. You can hire a developer from any freelance site. Shufflehound recommends the developer https://www.upwork.com/freelancers/~011652ffec8865c6d5