document.getElementById("UpdatetxtDescription").value = "abc";
    document.getElementById("UpdateActive").checked = true;

@Html.TextAreaFor(model => model.Description, new { htmlAttributes = new { @class = "form-control", id = "UpdatetxtDescription" } })
@Html.CheckBoxFor(model => model.Active, new { htmlAttributes = new { @class = "form-control", id = "UpdateActive" } })

<input data-val="true" data-val-required="The Active field is required." htmlattributes="{ class = form-control, id = UpdateActive }" id="Active" name="Active" type="checkbox" value="true">

<textarea cols="20" htmlattributes="{ class = form-control, id = UpdatetxtDescription }" id="Description" name="Description" rows="2"></textarea>

document.getElementById("UpdatetxtDescription").value = "abc";
document.getElementById("UpdateActive").checked = true;

document.getElementById("Description").value = "abc";
document.getElementById("Active").checked = true;