How to replace the value in a textbox that retrieve from database

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Javascript

HTML

database

Greetings,
I wish to replace the value retrieve from database in a textbox for data update. However, my codes cannot work properly and I don't know the reason. Can some one help me to look on it? Appreciate the help very much.

What I have tried:

<label for="Emp_Type" class="col-sm-2 col-sm-push-1 control-label">Employee Type <span> * </span> </label>
<div class="col-sm-2" style="padding-left:8%;width:250px">
<select class="form-control" name="type" id="type" onChange="changetextbox()"/>
</select>
</div>

<input type="text" class="form-control" style="width:120px;padding-left:1%" id="pay" name="pay" value="<?php echo $payType?>" readonly required>


function changetextbox()
{
if (document.getElementById("type").value === "Full Time")
{
//replace value of Pay Type
var str = document.getElementById("pay").value = "";
var res = str.replace("", "Hourly");
document.getElementById("pay").value = res;
}
}