Depending on the Site Template you used and features that are activated sometimes the "Navigation Elements" link is not visible on the "Site Settings" Page. But you can always navigate there manually. Add
/_layouts/15/navoptions.aspx to the end of your site url, for example,
You should see the following page with just two options:
Then just uncheck the "Enable Quick Launch" checkbox and click "OK".
Go to the All Site Settings page, and under the "Look and Feel" section, choose the option titled "Change the look" -- this will take you to the "_layouts/15/designgallery.aspx" page. If you do not actually want to change your colors, choose the first option titled "Current". This brings up another page (see screenshot below") that allows you to choose a new Layout -- choose "Oslo", and hit "Try it out", and eventually "Yes, keep it."