The first thing you need to know about Excel is that it's only compatible with Windows. Therefore, there are no other options but to install it on your computer. On the other hand, there are a lot of different types of Excel-like spreadsheets available for Mac, iOS, and Android devices. So, you can choose to download one of them. 

The next thing you need to know is that Trello is a tool to organize and share your work in the form of cards and lists. Therefore, you need to install its app as well.

To export a board to Excel, first, click on it in the view on your computer. Then, in the menu, you will see a button that says "Export to Excel." Now, choose the file type of your choice from the drop-down menu and it will be exported to Excel. 

As you can see, this is a simple tool to save Excel files.