To start, click on the dashboard (on the left-hand side panel) containing the chart you want to edit, as shown below.
Next, click the three dots in the top right on the chart you want to edit, as shown below.
From here, click Edit on the dropdown menu.
Next, click Options on the right-hand side panel, as shown below.
Now, you will be able to toggle the Show legend button either off or on by selecting the blue slider, as shown below.