Hello,
You can change permissions with the following commands.
browse to your installation directory ex. cd /var/www/nextcloud
then sudo find . -type f -print0 | sudo xargs -0 chmod 0640 for files
and sudo find . -type d -print0 | sudo xargs -0 chmod 0750 for directories
https://www.makeuseof.com/tag/chmod-command-linux-file-permissions/