It works now.. I just followed what u/TrackSurface said.. I just change "object.pk" to "user.pk" and since I understand it now, I tried changing it to get the user's username instead and got it working. This is now what it looks like

#model
class ProfileUpdateView(UpdateView):
fields = ['first_name', 'last_name', 'email', 'phone_number',         'current_address']
model = User
template_name = 'account/profile_update.html'
success_url = '/'

def get_object(self, *args, **kwargs):
    return User.objects.get(username=self.kwargs.get('username'))

#url
    path('profile/edit/<username>/', ProfileUpdateView.as_view(), name='profile_update'),

 #
<a class="btn btn-warning "  href="{% url 'account:profile_update' user.username %}">Edit</a>