The clear and start over tool won't work since you've already registered your account. You have two options:
1. You can go back through the return and manually update all 2015 entries to 2016. If you just had a 2015 W-2 this would be pretty easy. You just return to the income section (or type W-2 in the search box and select the Jump to W-2 that comes up) and select delete next to the 2015 W-2. Then add the 2016.
2. You can create another account and start fresh.
