In order to change the activity type on an existing activity, login to your account on Runkeeper.com, go to your "Me" tab, select the activity, then click on the drop down menu in the top right corner of your screen. Once you click on it you'll see an "Edit Activity" option. Just click that and you'll be able to edit your activity type!
Once on the activity edit page, scroll down to the Activity Data section, where you can select different activity types. After you've made the necessary edits, scroll to the bottom of the page and click Save, at which point your activity data will be updated.
**You cannot change the activity type of an existing activity on the mobile app.**