This depends on the different type you have purchased. Many bubble options are able to have a quick wipe down with some soapy water, which can make cleaning quick and easy. Others, however, offer the options of removable pads or machine washable options.
As always, this comes down to each, different purchase. As such, you should always follow the recommended guidelines given by the manufacturer, in order to ensure you won't accidentally ruin the integrity of the cat backpack.