Controller/Adminhtml/FastlyCdn/CustomSnippet/EditCustomSnippet.php
public function execute()
    {
        $result = $this->resultJson->create();
        try {
            $name = $this->getRequest()->getParam('name');
            $type = $this->getRequest()->getParam('type');
            $priority = $this->getRequest()->getParam('priority');
            $vcl = $this->getRequest()->getParam('vcl');

            $validation = $this->config->validateCustomSnippet($name, $type, $priority);
            $error = $validation['error'];
            if ($error != null) {
                throw new LocalizedException(__($error));
            }
            $snippetName = $validation['snippet_name'];
            $fileName = $type . '_' . $priority . '_' . $snippetName . '.vcl';

            $write = $this->filesystem->getDirectoryWrite(DirectoryList::VAR_DIR);
            $snippetPath = $write->getRelativePath(Config::CUSTOM_SNIPPET_PATH . $fileName);

            $write->writeFile($snippetPath, $vcl);

            return $result->setData([
                'status'    => true
            ]);
        } catch (\Exception $e) {
            return $result->setData([
                'status'    => false,
                'msg'       => $e->getMessage()
            ]);
        }
    }
public function execute()
    {
        $result = $this->resultJson->create();
        try {
            $name = $this->getRequest()->getParam('name');
            $type = $this->getRequest()->getParam('type');
            $priority = $this->getRequest()->getParam('priority');
            $vcl = $this->getRequest()->getParam('vcl');

            $validation = $this->config->validateCustomSnippet($name, $type, $priority);
            $error = $validation['error'];
            if ($error != null) {
                throw new LocalizedException(__($error));
            }
            $snippetName = $validation['snippet_name'];
            $fileName = $type . '_' . $priority . '_' . $snippetName . '.vcl';

            $write = $this->filesystem->getDirectoryWrite(DirectoryList::VAR_DIR);
            $snippetPath = $write->getRelativePath(Config::CUSTOM_SNIPPET_PATH . $fileName);

            $write->writeFile($snippetPath, $vcl);

            return $result->setData([
                'status'    => true
            ]);
        } catch (\Exception $e) {
            return $result->setData([
                'status'    => false,
                'msg'       => $e->getMessage()
            ]);
        }
    }
Edit custom snippet

@return $this|\Magento\Framework\App\ResponseInterface|\Magento\Framework\Controller\ResultInterface