Editing django-rest-framework serializer object before save

I want to edit a django-rest-framwork serializer object before it is saved. This is how I currently do it -

def upload(request):
    if request.method == 'POST':
        form = ImageForm(request.POST, request.FILES)
        if form.is_valid(): # All validation rules pass
             obj = form.save(commit=False)
             obj.user_id = 15
             obj.save()

How can I do it with a django-rest-framework serializer object?

@api_view(['POST','GET'])
def upload_serializers(request):
    if request.method == 'POST':
         serializer = FilesSerializer(data=request.DATA, files=request.FILES)
         if serializer.is_valid():
              serializer.save()