@WarpinWolf Never overwrite any files below apps/cms_pico, all your changes will be lost as soon as the app is updated. If you want to edit index.twig, you basically want to create a own theme based on Pico’s default theme. Simply copy apps/cms_pico/appdata/themes/default to data/appdata_ocvw3gjxp4ow/cms_pico/themes/my-theme, make all changes you want and enable your new theme using the app’s admin interface. data/appdata_ocvw3gjxp4ow/cms_pico/themes/default can be deleted - it’s a leftover from the upgrade.
@ludovicis @Cult (named daita on GitHub) is the original author of this app. I took over development. You can find the install instructions in the first two posts of this thread.