The file system is 'read only,' you cannot make changes to a running container.
In order to upgrade a plugin, etc. you need to update the plugin in your local environment and then commit it to git. Once you push to Platform.sh, it will be build and deployed.
We suggest using Composer to manage your WordPress plugins and themes.
A guide for deploying WordPress to Platform.sh using Composer can be found here.
A guide for deploying vanilla WordPress to Platform.sh without Composer can be found here.