Great question! No action is needed from you to "reset" the app for 2018--you'll continue to record your deductions in 2018 normally, and your app will save all of your deduction information from last year.
The "Total tax deduction" number that you see on your Tax Savings screen is meant to reflect all of your deductions to date, not just for 2017 alone.
However, when you export your reports, those reports will only reflect deductions from one tax year. You'll get to choose the tax year you'd like to export!
To do so, you can go to the menu of your app, click the "Send my tax report" button, and choose the tax year that you'd like to view.
Here's what the export process will look like: