Change the blending mode of artwork

Select an object or group (or target a layer in the Layers panel).
If you want to change the blending mode of a fill or stroke, select the object, and then select the fill or stroke in the Appearance panel.
In the Transparency panel, choose a blending mode from the pop‑up menu.
You can isolate the blending mode to a targeted layer or group in order to leave objects beneath unaffected. To do this, select the target icon to the right of a group or layer in the Layers panel that contains an object using a blending mode. In the Transparency panel, select Isolate Blending. (If the Isolate Blending option isn’t visible, select Show Options from the Transparency panel menu.)

Group (star and circle) with Isolate Blending option deselected (left) compared to selected (right)

To select all objects that use the same blending mode, select an object with that blending mode, or deselect everything and choose the blending mode in the Transparency panel. Then choose Select > Same > Blending Mode.
For a video on working with blending modes, see www.adobe.com/go/vid0055.