By default you can choose a date range that's relative to today's day (i.e. the last, day, week, month etc).
But if you want to input a custom date range (i.e. 03-07-17 to 5-07-17) then you need to change the picker style.
Please note: The option to change style is only available in the Desktop version of PowerBI. Please ensure you have the latest version (at least Version: 2.48.4792.721)
You can do this by hovering over the date range box and selecting the small menu icon to reveal a list. Now choose 'between'.
To change back to 'relative' simply select it from the the list using the same process described above.
If you want to keep the date range a certain style, then save your report and it'll remember your preference.
At the time of writing, the ability to change date picker style is a PowerBI Desktop only feature. However, if you make your required changes in the Desktop version, save you report, and upload to PowerBI Service, then your changes will be visible there too.