[Django]-Django update table using data from another table

You cannot use F, but you can use Subquery and OuterRef:

from django.db.models import Subquery, OuterRef

cost = Category.objects.filter(
    id=OuterRef('category_id')
).values_list(
    'price_markup'
)[:1]

Product.objects.update(
    new_cost=Subquery(cost)
)

Note: My answer is outdated now, Django 1.11 introduced OuterRef which implements this feature. Check Andrey Berenda answer.

According to the documentation, updates using join clauses are not supported, see:

However, unlike F() objects in filter and exclude clauses, you can’t
introduce joins when you use F() objects in an update – you can only
reference fields local to the model being updated. If you attempt to
introduce a join with an F() object, a FieldError will be raised:

# THIS WILL RAISE A FieldError
>>> Entry.objects.update(headline=F('blog__name'))

Also, according to this issue, this is by design and there is no plans to change it in the near future:

The actual issue here appears to be that joined F() clauses aren’t
permitted in update() statements. This is by design; support for joins
in update() clauses was explicitly removed due to inherent
complications in supporting them in the general case.

rows = Product.objects.filter(old_field__isnull=False)
for row in rows:
     row.new_field = row.old_field.subfield
Product.objects.bulk_update(rows, ['new_field'])