How can I change form field values after calling the is_valid() method?

Save the model form with commit=False, then modify the instance before saving to the database.

if form.is_valid() and form1.is_valid():
    instance = form1.save(commit=False)
    instance.uid = '12134324231'
    instance.save()

If form1 had any many-to-many relationships, you would have to call the save_m2m method to save the many-to-many form data. See the docs for full details.

From Overriding clean() on a ModelFormSet.

Also note that by the time you reach this step, individual model instances have already been created for each Form. Modifying a value in form.cleaned_data is not sufficient to affect the saved value. If you wish to modify a value in ModelFormSet.clean() you must modify form.instance:

from django.forms import BaseModelFormSet

class MyModelFormSet(BaseModelFormSet):
    def clean(self):
        super(MyModelFormSet, self).clean()

        for form in self.forms:
            name = form.cleaned_data['name'].upper()
            form.cleaned_data['name'] = name
            # update the instance value.
            form.instance.name = name

So what you should do is:

if form.is_valid() and form1.is_valid():
        form1.instance.uid ='12134324231'
        form1.save()