Since you are looking for a string as the result
var $div = $('<div />',{html: content});
$div.find('option[value="Maintenance Plus - Stand Alone form"]').attr('selected', 'selected');
content = $div.html();
console.log(content)
Demo: Fiddle
Question
I have the next string variable which contains the next string
var content = '<div id="TypeModifiedProduct" class="form-group WSUage2">
<label for="TypeModified">Type</label>
<span class="pull-right alignText" style="margin-right: 40%;">
<input type="checkbox" id="UseTypeModified" name="UseTypeModified" > Use option
</span>
<select name="TypeModified" id="TypeModified" class="form-control" style="width:40%" >
<option value="Maintenance - Stand Alone form" >Maintenance - Stand Alone form</option>
<option value="Maintenance Plus - Stand Alone form">Maintenance Plus - Stand Alone form</option>
</select>
</div>';
Now I want for example search for the second option
<option value="Maintenance Plus - Stand Alone form">Maintenance Plus - Stand Alone form</option>
And change it for beign selected option
<option value="Maintenance Plus - Stand Alone form" selected >Maintenance Plus - Stand Alone form</option>
Then finally return this new string with the change in the option element.
Thanks in advice
Solution
Since you are looking for a string as the result
var $div = $('<div />',{html: content});
$div.find('option[value="Maintenance Plus - Stand Alone form"]').attr('selected', 'selected');
content = $div.html();
console.log(content)
Demo: Fiddle