Q:

change the "end" attribute on an Aura:iteration using a button/javascript

I am trying to create a load more button that when clicked changes how many iterations are in an Aura:iteration.  Right now the End attribute on that iteration is 10, and we would like it to change by increments of 5 whenever the load more button is pressed.  Here is my code so far:
    cardsLoaded: function(component, event, helper){
   component.getElementByTagName("aura:iteration").setAttribute("end", end +5);
},

Button:
<lightning:button label="Load More" title="Neutral action" onclick="{!  c.cardsLoaded}"/>

A:

You can't use document.getElementByTagName, because components are "virtual" elements, not real elements (LWC does expose them like this, but that's a different story).  Here's one possible implementation, by setting an attribute, self-contained to make it easier to understand:
<aura:application extends="force:slds">
    <aura:attribute name="end" default="5" type="Integer" />
    <aura:attribute name="values" default="[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]" type="List" />
    <lightning:button onclick="{!c.add5}" label="+5" />
    <aura:iteration items="{!v.values}" end="{!v.end}" var="item">
        <div>
            {! item}
        </div>
    </aura:iteration>
</aura:application>

({
    add5: function(component, event, helper) {
        component.set("v.end", component.get("v.end") + 5);
    }
})

You can also use component.find:
<aura:application extends="force:slds">
    <aura:attribute name="values" default="[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]" type="List" />
    <lightning:button onclick="{!c.add5}" label="+5" />
    <aura:iteration aura:id="list" items="{!v.values}" end="5" var="item">
        <div>
            {! item}
        </div>
    </aura:iteration>
</aura:application>

({
    add5: function(component, event, helper) {
        var list = component.find("list");
        list.set("v.end", list.get("v.end") + 5);
    }
})

