How do I change beginning weight?

Go to Home, Check-in then scroll down to see the link that says "edit previous entries".

No, I meant scroll on the Check-in page to the link that says edit previous entries, then chose that. When that page comes up there should be numbered pages where all your previous entries are listed. Choose highest page number and click it. Then find your first entry. They are listed by date entered.

You must use the Web version, not the mobile apps, to update your starting weight.

Navigate to APPS > Tickers > Update your weight data. Enter your desired starting weight and click Save Changes. That will update the oldest date/weight in your profile.

Alternately, navigate to MY HOME > Check-in > Edit Previous Entries.
a. Enter a new value in the Starting Weight text box and click Change; or
b. Scroll down and click on the highest page number to take you to the oldest date/weight entry. Once you find your oldest date/weight entry, under Add New Entry, enter the new date/weight and click Add New Entry. That will replace the oldest date/weight with your new one.
c. If you want to keep all of your historical data, Add New Entry at least one day older than the oldest date/weight.