How to Update ImageField in Django?

I think you missed the enctype="multipart/form-data", try to change:

<form method="POST" action="/myapp/updateimage/{{ singleimagedata.id }}">

into;

<form method="POST" enctype="multipart/form-data" action="{% url 'updateimage' id=singleimagedata.id %}">

Don't miss also to add the image_color field to your html input.
Because, in your case the image_color field model is designed as required field.

To remove & update the old image file from directory;

import os
from django.conf import settings

# your imported module...


def updateimage(request, id):  #this function is called when update data
    old_image = ImageModel.objects.get(id=id)
    form = ImageForm(request.POST, request.FILES, instance=old_image)

    if form.is_valid():

        # deleting old uploaded image.
        image_path = old_image.image_document.path
        if os.path.exists(image_path):
            os.remove(image_path)

        # the `form.save` will also update your newest image & path.
        form.save()
        return redirect("/myapp/productlist")
    else:
        context = {'singleimagedata': old_image, 'form': form}
        return render(request, 'demo/editproduct.html', context)

I had a similar issue while updating the profile_pic of user. I solved this with the following code I think this might help:

Models.py

class Profile(models.Model):
    # setting o2o field of user with User model
    user_name = models.OneToOneField(User, on_delete=models.CASCADE, blank=True, null=True)
    first_name = models.CharField(max_length=70, null=True, blank=True)
    last_name = models.CharField(max_length=70, null=True, blank=True)    
    profile_pic = models.ImageField(upload_to="images", blank=True, null=True,)


    def __str__(self):
        return str(self.user_name)

forms.py

class ProfileEditForm(ModelForm):
    class Meta:
        model = Profile
        fields = '__all__'
        # excluding user_name as it is a one_to_one relationship with User model
        exclude = ['user_name']

views.py

@login_required(login_url='login')
def edit_profile(request, id):
    username = get_object_or_404(Profile, id=id)
    extended_pro_edit_form = ProfileEditForm(instance=username)
    if request.method == "POST":
        extended_pro_edit_form = ProfileEditForm(request.POST, request.FILES, instance=username)
        if extended_pro_edit_form.is_valid():
            extended_pro_edit_form.save()
            next_ = request.POST.get('next', '/')
            return HttpResponseRedirect(next_)

    context = {'extended_pro_edit_form': extended_pro_edit_form}
    return render(request, 'edit_profile.html', context)

edit-profile.html

<form action="" method="post"
              enctype="multipart/form-data">
            {% csrf_token %}
            {{ extended_pro_edit_form.as_p }}
            {{ extended_pro_edit_form.errors }}
            <!--To redirect user to prvious page after post req-->
            <input type="hidden" name="next" value="{{ request.GET.next }}">

            <button type="submit">UPDATE</button>

        </form>

Answer from @binpy should solve your problem. In addition to your second answer, you could do:

def updateimage(request, id):  #this function is called when update data
    data = ImageModel.objects.get(id=id)
    form = ImageForm(request.POST,request.FILES,instance = data)
    if form.is_valid():
        data.image_document.delete()  # This will delete your old image
        form.save()
        return redirect("/myapp/productlist")
    else:
        return render(request, 'demo/editproduct.html', {'singleimagedata': data})

Check delete() method on django docs.

some times something like cached old image is not replaced in the front-end so you might just need to forces refresh by pressing CTRL + F5 or clear your browsing history.

the answer given by @binpy is a needed update so that the files are passed to the back-end.