Как заменить иконкой FontAwesome input?

<button onclick="selectAll(this)" id="select_btn"> Select All</button>  
<form method="post" action="block.php"> 
  <div class="inputs"> 
    <button type="submit" name="block_user"><i class="fas fa-lock"></i></button>  
    <input type="submit" value="Unblock" name="unblock_user"> 
    <input type="button" value="LogOut" onClick='location.href="login.php"'> 
  </div>   
  <div class="table"> 
    <table> 
      <tr> 
        <th></th> 
        <th>Name</th> 
        <th>Date of registration</th> 
        <th>Last login</th> 
        <th>Status</th> 
      </tr>         
    </table> 
  </div> 
</form>