You can simply click on Edit and change the shareholder name.

The only thing to note here is, if this is an investor from your "Connections" then these shares will be visible in their account. If you change the name to a custom name, the green tick will disappear and whilst you will see the shares in your captable, it will not link to the investor profile.

A work around would be to click on the Edit and scroll to the Notes sections at the bottom and specify the company they are assigned to for example.