Only labels that are on Onhold and Draft status could be edited. Here's how.

1. Upon logging in, select Shipping Labels.

2. Click Generate Labels.

3. Select either On hold or Draft. By doing this, you are limiting the list that will appear.

4. At the last column, you must these buttons.

Choose Edit button.

5. You will be directed to Edit Generate Label.

This page should be familiar to you. 

You can now start doing corrections or update your Shipmark.