Summary
To change the order of your portfolio is very simple. You just have to change the dates of their publishing. But since that could extremely annoying to do, there is luckily an awesome plugin.
Change the dates
In case you only want to move just one portfolio or only a few, I would recommend just changing the dates. Every plugin is more code, more potential risk, slower website. You can easily change the date with the quick edit button, if you hover over the portfolio in your WordPress.

Post Type Order Plugin
If you really want to play with the order of your portfolio, luckily, there is this amazing plugin. So, all you have to do is install, activate and save settings.
https://wordpress.org/plugins/post-types-order
Install and Activate
So search for Post Type Order and install and activate this plugin.

Save the settings
All you have to do is to go to Settings -> Post Type Order -> Press the Save Settings button at the bottom.


Drag and Drop the Portfolio
And now, you should see these arrows at the portfolio, you can now drag the portfolios and move them up and down. In case you will not see the effect, try what is called hard refresh by pressing Cntrl/Cmd + Shift + R (letter R as refresh).

Good luck