It all depends upon how you set up the voice. If the Mandolin voice is attached to a default style, the effects have been set and saved within the style file. Therefore, the easiest way to make the changes would be with a registration
First, select the Mandolin voice as the R1 voice. Now,Press the direct access button and press the Voice Effects button. This should show you the effects list for that voice. Select Trill. You can also select the speed, volume and other needed parameters. You can save this to a registration, or run it through the onboard Style Creator program, save the mandolin to the style as an OTS, then save the style as a custom style.
I don't own a PSR-S910, therefore I'm working blind with this response.
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)