First, go to the Cap table page located on the left navigation bar under Essentials → Cap Table.
Editing individual securities
Select the certificate I.D. of the security you wish to edit (in the 'Certificate' column),
When viewing the security details select Actions → Edit.
Editing securities in bulk
Learn more about spreadsheet mode here.
Hit the green Actions button → Edit.
This will open the spreadsheet mode where you can make additional edits.