You cannot delete your plan, but when you create a new plan, your current plan will be archived.
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 the link at the bottom of the screen to create a new plan.
4) Accept the dialog that your current plan will be archived*
5) Enter your race information
If you would like to see your old plan you can tap the More tab, then Past Training Plans for a complete list of your archived plans.
*There is no way to restore an archived plan.