Finding layers and updating property of a layer

const markerThatTitleIsReearth = reearth.layers.find(
    layer => layer.type === "marker" && layer.title === "Re:Earth"
);

if (markerThatTitleIsReearth) {
    // temporally update marker location
    reearth.layers.overrideProperty(markerThatTitleIsReearth.id, {
        default: {
            location: { lat: 100, lng: 0, height: 0 }
    }
    });
}

Note that the result of reearth.layers.overrideProperty will not be persisted. If you close or reload the page, the changes will be undone.

The details of property object are not documented yet, but you can get the object of the current layer by running reearth.layers.layers in the console of the developer tools. By looking at the properties in this object, you can find out what properties can be overridden.