Open Control Panel, and then click on "Change adapter settings" (this should appear on the left side pane). Right click on Wi-fi (or the equivalent name), and click on Properties. In the new dialog box, click on "Configure...", which will lead you to a dialog box with multiple tabs. Click on Advanced, and select "Preferred Band". Within the dialog box, on the right side, you will find 3 preferences:
1. No Preference
2. Prefer 2.4 Ghz
3. Prefer 5 Ghz
Select your preference and click OK until you close all the dialog boxes. Your wifi adapter should reset and connect to your preferred bandwidth.
This is how I connect to 5 Ghz automatically on my Windows 10 laptop. Settings and nomenclature could change with different versions of Windows, or even with different wifi adapters, but I think this is a common process.
Hope it works for you!