src/Forms/ShopAccountForm.php
public function proceed($data, $form, $request)
    {
        $member = Security::getCurrentUser();
        if (!$member) {
            return false;
        }

        $form->saveInto($member);
        $member->write();
        $form->sessionMessage(_t(__CLASS__ . '.DetailsSaved', 'Your details have been saved'), 'good');

        $this->extend('updateShopAccountFormResponse', $request, $form, $data, $response);

        return $response ?: $this->getController()->redirect(CheckoutPage::find_link());
    }
public function proceed($data, $form, $request)
    {
        $member = Security::getCurrentUser();
        if (!$member) {
            return false;
        }

        $form->saveInto($member);
        $member->write();
        $form->sessionMessage(_t(__CLASS__ . '.DetailsSaved', 'Your details have been saved'), 'good');

        $this->extend('updateShopAccountFormResponse', $request, $form, $data, $response);

        return $response ?: $this->getController()->redirect(CheckoutPage::find_link());
    }
Save the changes to the form, and redirect to the checkout page

@param array       $data
@param Form        $form
@param HTTPRequest $request

@return bool|HTTPResponse
@throws \SilverStripe\ORM\ValidationException