You just need to use an if..else
statement:
function RandomID() {
var value;
var rnd = Math.floor(Math.random() * 11);
if (rnd === 7)
value = "Wassup";
else if (rnd <= 5)
value = "Hello";
else
value = rnd;
document.getElementById('id').value = value;
}
<button class="button" onclick="RandomID();" style="font-family: sans-serif;">RUN</button>
<input class="input" type="text" id="id" name="id" size="3" readonly />
solved HTML random number to text