can I update a javascript variable being displayed using innerhtml

Question

I'm trying to build a basic budget calculator tool in javascript which displays a running total at the top of the page which updates when individual budget items are changed. To do this I'm using [document.getElementById("runningTotal").innerHTML += total_sp;]. This works fine for calculating the running total but every time I make a change the new total is appended to the old total, so the output on screen makes no sense. Can anyone tell me how I can do this so that the running total updates every time a change is made replacing the old total with the new total? I'm new to all this, so very grateful for any help you can offer. Cheers!

<script>
    // defining pre-change spending levels
    var research_sp = 217200
    var wage_sp = 155000
    var total_sp = wage_sp + research_sp
    function changeresearchspending() {
        var nameElement = document.getElementById("researchcalc");
        //defining percentage change to research spending levels
        var research_r = nameElement.value;
        var research_ch = research_r * research_sp;
        var research_sp_end = research_sp + research_ch;
        total_sp = total_sp + research_ch
        document.getElementById("researchChange").innerHTML += research_ch;
        document.getElementById("endresearchTotal").innerHTML += research_sp_end;
        document.getElementById("endTotalSpend1").innerHTML += total_sp;
        document.getElementById("runningTotal").innerHTML += total_sp;
    }
    function changewagespending() {
        var nameElement = document.getElementById("wagecalc");
        //defining percentage change to wage spending levels
        var wage_r = nameElement.value;
        var wage_ch = wage_r * wage_sp;
        var wage_sp_end = wage_sp + wage_ch;
        total_sp = total_sp + wage_ch
        document.getElementById("wageChange").innerHTML += wage_ch;
        document.getElementById("endwageTotal").innerHTML += wage_sp_end;
        document.getElementById("endTotalSpend2").innerHTML += total_sp;
        document.getElementById("runningTotal").innerHTML += total_sp;
    }
</script>

<div id="runningTotal">
    SPENDING (RUNNING TOTAL) £
</div>
<br>
<div id="startresearchTotal">
    Baseline research spending is: £
    <script>document.write(research_sp)</script>
    <br><br>
    Baseline wage spending is: £
    <script>document.write(wage_sp)</script>
    <br><br>
    Baseline total spending is: £
    <script>document.write(total_sp)</script>
</div>
<br>
<input id="researchcalc" type="number">
<input type="button" value="change research spend" onClick="changeresearchspending()">
<div id="researchChange">
    <br> Change in research spending is: £
</div>
<div id="endresearchTotal">
    <br> New research spending is: £
</div>
<div id="endTotalSpend1">
    <br> New total spending is: £
</div>
</div>
<br>
<input id="wagecalc" type="number">
<input type="button" value="change wage spend" onClick="changewagespending()">
<div id="wageChange">
    <br> Change in wage spending is: £
</div>
<div id="endwageTotal">
    <br> New wage spending is: £
</div>
<div id="endTotalSpend2">
    <br> New total spending is: £
</div>

Answer 1

You can replace the += by =, like this:

document.getElementById("runningTotal").innerHTML = total_sp;