All you need to do is add an input of a "button" type immediately after your password field, and before the BReak element to keep it on the same line.
You may need to tweak your CSS to fine tune location, and you are going to have to add in some more
PHP
to actually do something when it is clicked on
<form>
<label for="username"></label>
<input type="text" name="username" placeholder="<?=$_SESSION['name']?>" id="username" disabled>
<br>
<label for="password"></label>
<input type="password" name="password" placeholder="<?=$password?>" id="password" disabled>
<!--
<input type="submit" name="ChangePW" value="Change">
<br>
</form>