var selected = component.get("v.key"); //key used for storing tab ids.
component.find("tabset").set("v.selectedTabId",selected);

<aura:application extends="force:slds" access="global">
    <aura:attribute name="key" type="String" default="two" />
    <aura:handler name="change" value="{!v.key}" action="{!c.selectTab}"/>

    <lightning:tabset aura:id="tabs" selectedTabId="two">
        <lightning:tab label="Item One" id="one">
            Sample Content One
        </lightning:tab>
        <lightning:tab label="Item Two" id="two">
            Sample Content Two
        </lightning:tab>
        <lightning:tab label="Item Three" id="three">
            Sample Content Three
        </lightning:tab>
    </lightning:tabset>

    <br/><br/><br/>

    <lightning:button variant="brand" label="ChangeValue" onclick="{!c.handleClick}" />
</aura:application>

({
    selectTab : function(component, event, helper) {
        var selected = component.get("v.key");
        component.find("tabs").set("v.selectedTabId",selected);
    },
    handleClick : function(component, event, helper){
        component.set("v.key","three"); // value assignation
    }
})