Nikandro,
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>CodePen - A Pen by Niksan</title>
<style>
*{
padding: 0;
margin: 0;
box-sizing: border-box;
}
.main{
background: rgb(224, 224, 224);
width: 100%;
display: flex;
}
.sidebar{
width: 15%;
height: 900px;
background-color: rgb(180, 180, 180);
}
.h1{
font-size: 1.5em;
padding: 5px;
padding-left: 15px;
}
.info{
background: rgb(255, 223, 211);
padding: 5px;
padding-left: 15px;
}
.table-knopki{
width: 100%;
height: 50px;
background: aliceblue;
display: flex;
}
.tab-text{
font-size: 2em;
font-weight: bold;
padding: 5px;
margin-left: 20px;
padding-right: 200px;
}
.tab{
width: 100%;
margin-top: 60px;
margin-left: 30px;
}
button{
padding: 5px;
border: 0;
color: azure;
font-weight: bold;
margin: 5px;
}
th,td{
border: 1px solid black;
text-align: center;
width: 30px;
height: 30px;
padding: 15px;
}
.first-n{
width: 200px;
height: 30px;
border: 1px solid black;
background: rgb(168, 167, 167);
}
.knopka {
background: orangered;
}
</style>
</head>
<body translate="no">
<div class="tab">
<div class="table-knopki">
<p class="tab-text"> Table</p>
<button class="knopka green" onclick="add()">Add day</button>
<button class="knopka red" onclick="remove()">Remove Day</button>
<button class="knopka blue">Update Table</button>
</div>
<table>
<tr id="myTD">
<td class="first-n"></td>
<td>Average</td>
</tr>
<tr>
<td>Student 1</td>
<td>3.0</td>
</tr>
<tr>
<td>Student 2</td>
<td>3.0</td>
</tr>
<tr>
<td>Student 3</td>
<td>3.0</td>
</tr>
</table>
</div>
<script>
function add() {
var para = document.createElement("TD");
[...document.querySelectorAll("tr")].forEach(tr => tr.append(para.cloneNode()))
};
function remove() {
[...document.querySelectorAll("tr td:last-child")].forEach(td => td.remove());
}
</script>
</body>
</html>