Like you mentioned the firmware is really challenging and important. Sometimes, there are security update in the firmware which you need to deploy as soon as possible. However, it is good idea to check with manufacturers (Dell and Lenovo) in your case for best practices. Make sure take a complete backup and be ready for format and reinstall or replace the device. Start doing it slowly with small group of devices. Make devices are fully charge and they are connected to the power adaptor and your electricity is stable.