There is no way to reset or restart your exact training plan because is optimized for the date range you had when you created that plan.
But, if you enter in a new goal race a new personalized plan will be generated for you starting today building up to your race date.
To create a new plan you will need to archive your current plan and reenter your race information. Follow these steps to create a new plan:
1) Tap the plan personalization icon in the upper right corner of the app
2) Tap the Goal Race section
3) Tap the pink New Goal Race button
4) Accept the dialog that your current plan will be archived
5) Enter your race information