You can add your user to the www-group with:

sudo usermod -a -G www-data example

where the user is example.

Once you’ve done this, with the default permissions (775 for folders) your user will have access to read/write files owned by the www-data group. Now update your file ownership with:

chown -Rf www-data:www-data /var/www/html

where /var/www/html is your web root.