How can I update shares in my holdings tab?

Clicking on the number of shares in your holdings will enable you to edit that lot's information.

Then simply select the action you would like to take.

In this case I am editing my number of shares to account for the ones I sold.

Click save lot and you're done.

PRO TIP - keep track of the performance of shares you sold by creating a separate lot and selecting Sell Short instead of Buy.