Using ajax in thymleaf, update only part of form - thymeleaf

Question

I use SpringBoot, Thymeleaf.
I want to update the value only for part of form in update page.
<form id="form" class="needs-validation col-sm-6" th:action="#{'/user/edit/' + ${userInfo.id}}" th:object="${userInfo}" method="post" novalidate>
<input type="hidden" name="_method" value="put"/>
<input type="hidden" name="id" th:value="${userInfo.id}"/>
<div class="form-group">
<label for="name">Name</label>
<h4 id="name" th:text="${userInfo.name}" th:value="${userInfo.name}"></h4>
<input type="hidden" name="name" th:value="${userInfo.name}"/>
</div>
<hr>
<div class="form-group">
<label for="nickname">Nickname</label>
<input id="nickname" type="text" name="nickname" th:value="${userInfo.nickname}" placeholder="enter nickname" required minlength="2">
</div>
<hr>
<div class="form-group">
<label for="volunteerTime">Volunteer Time</label>
<span id="volunteerTime" th:text="${userInfo.volunteerTime}+'hour'" name="volunteerTime" th:value="${userInfo.volunteerTime}"></span>
<input type="text" size="5px" placeholder="hour"/>
<button type="button" name="action" value="plus" onclick="ajax()">plus</button>
<button type="button" name="action" value="minus" onclick="ajax()">minus</button>
</div>
<div class="form-group">
<input type="submit" value="update">
</div>
</form>
The "volunteerTime" section of the above code seeks to update and show values added to existing data according to the number entered.
The same goes for Minus.