HERE IS HOW:
Step 1: Please make sure that you have chosen manual log as your data source on your Metrics page:
Step 2: Go to the Manual Log page - all metrics that haven't been set up or have the Manual log as their data source will be displayed on the list:
Choose the metric you would like to enter data for and fill in all fields. Click on Add Data button.
You may also edit or remove your log if necessary:
IMPORTANT. PLEASE READ CAREFULLY:
When you join a contest with the forbidden manual log, any data logged manually will not be reflected in the contest’s results and will be shown as null.
PLEASE REMEMBER TO SET YOUR DEVICE BACK AS A DATA SOURCE IF YOU ARE PLANNING TO CONTINUE USING YOUR DEVICE ONCE THE MANUAL ENTRY IS COMPLETED.
Here are the details:
If you are an Apple/Fitbit/Garmin/iHealth/Withings/Misfit/TomTom/Polar user and need to update your data manually, please note that once you put your device back, it will overwrite your information unless the 1-day period is over (we pull your 1-day retrospective data with these vendors).
So, for example, if you want to log data for June 18, you need to wait until June 19 for it not to be overwritten by, say, Fitbit.
If the missing dates are more than 1 day, you should be fine to log everything manually for that period and then switch back to your device.