To change your default crate:

Select the crate that you wish to make the default.

Select Set as Default Crate from the Right-click pop-up menu.
Clicking the Buy button for a particular item will result in the item being added to your default crate.
The default crate is distinguished from the others by the icon.
Select the crate that you wish to make the default.
Select Set as Default Crate from the Right-click pop-up menu.