At the top of the dashboard, you will see the following buttons and options. (If you don't have multiple portfolios, you won't see the 3 left buttons / options, you will only see the "Manage Portfolios" text).
Select "Manage Portfolios".
Select the portfolio you want to rename.
Once you've selected the portfolio, click on the gear icon in the top right corner. It looks like a wheel with spokes on it.
You will then see a settings page where you can click the pencil icon to change the name of the portfolio.
Type in the new portfolio name and hit enter.
That's it! You have now renamed your portfolio. This can be changed at any time and will instantly propagate to the rest of the application.