You can find the answers to a lot of this type of question in the Frequently Asked Questions (faq) link located at the very bottom of the page.
"Can I change my last weigh in entry if I made a mistake?
Yes, if you have made a mistake entering your weight information for the current date, proceed to weigh in again and you will be prompted to confirm that you would like to overwrite the last entry. Click "Ok" to then enter the correct information."
Someone who is busier than you is working out right now.
There will come a day when you can no longer do this. Today is not that day.