You can edit your Watchlists by tapping EDIT on the top left corner of your Watchlist. 
From the Edit view, you can add symbols by searching for them, or remove symbols by tapping the red symbol on the left, or reorder them by tapping and dragging on the three lines to the right. To delete or rename the Watchlist, you can use the buttons at the bottom of the page.
Tap Done on top right when you are finished editing your Watchlist. 