def __init__(self, *args, **kwargs):
    super(TagForm, self).__init__(*args, **kwargs)
if 'instance' in kwargs:
    self.fields['projects'].initial = self.instance.project_set.all()

def save(self, *args, **kwargs):
    super(TagForm, self).save(*args, **kwargs)
self.instance.project_set.clear()
for project in self.cleaned_data['projects']:
    self.instance.project_set.add(project)
Редактирование обеих сторон M2M на странице администратора