To update Magento 2.0.x **with sample data**, I followed these steps (also works to update to 2.1 RC):

0. Apply [this commit](https://github.com/magento/magento2/commit/1919e7ed823c24f1cfac9e56efa1b691684e03dd) manually (it's a bugfix for the sample data that is released with 2.1)
1. Reset the sample data modules in Magento

        bin/magento sampledata:reset

 - if you get an ErrorException about not found composer.json files, see: http://magento.stackexchange.com/questions/120596/bin-magento-sampledatareset-tries-to-read-nonexistent-composer-json
2. Update composer.json without immediately updating dependencies (this would result in a conflict because the old magento module versions are still locked). For 2.0.7:

        composer require --no-update magento/product-community-edition ~2.0.7

    or for the latest 2.1 release candidate

        composer require --no-update magento/product-community-edition 2.1.0-rc3

3. **[only if you update to >=2.1]** Remove the sample data modules from `composer.json`:

        "magento/module-bundle-sample-data": "100.0.*",
        "magento/module-theme-sample-data": "100.0.*",
        "magento/module-widget-sample-data": "100.0.*",
        "magento/module-catalog-sample-data": "100.0.*",
        "magento/module-customer-sample-data": "100.0.*",
        "magento/module-cms-sample-data": "100.0.*",
        "magento/module-catalog-rule-sample-data": "100.0.*",
        "magento/module-sales-rule-sample-data": "100.0.*",
        "magento/module-review-sample-data": "100.0.*",
        "magento/module-tax-sample-data": "100.0.*",
        "magento/module-sales-sample-data": "100.0.*",
        "magento/module-grouped-product-sample-data": "100.0.*",
        "magento/module-downloadable-sample-data": "100.0.*",
        "magento/module-msrp-sample-data": "100.0.*",
        "magento/module-configurable-sample-data": "100.0.*",
        "magento/module-product-links-sample-data": "100.0.*",
        "magento/module-wishlist-sample-data": "100.0.*",
        "magento/module-swatches-sample-data": "100.0.*",
        "magento/sample-data-media": "100.0.*",
        "magento/module-offline-shipping-sample-data": "100.0.*",

4. Run the actual composer update:

        composer update

5. **[only if you update to >=2.1]** Reinstall sample data (you might need to apply the patch from (2) again):

        bin/magento sampledata:deploy