The Swatches panel / palette stores all of your colors, patterns and gradients. To edit any of them, simply double click one or select it and go to the gear symbol and click ‘Edit’. When you double click a gradient, the gradient editor will appear. In here you can change the gradient from a radial to a linear gradient which will then allow you to change the angle. You can see any changes you make update live in the document via the preview button. Double click the color handles to change colors, click anywhere below the color bar to add a new handle, select a handle and click the trash can to delete a handle. The opacity of a handle can be changed here too. When you are finished you can apply it to the selected object immediately.