You can delete the activity with the following instructions:

Sign into your account on a web browser at grokker.com
Access your "My Profile" by clicking on your profile circle icon in the top right corner of the the home page.
Click on the tab "I Did This" and locate the activity you would like to remove
To far right of an I Did This post, you should see an X. Click on the X to delete the I Did This post

If you wish to self report a different activity, you can go back and report again on that day. Please ensure if you had another activity on that day that your self report does not overlap.
If you need further assistance, please email support@grokker.com.