Hi,

If you need to set another journal as default journal from the user interface, you can do it by activating the developer mode.

Activate the debug mode, select the required journal in the field, click the debugger button near logged in user name, click on Set Default -> From the Default field in the opening pop up choose the Journal and click on save.

Once Click on Set Defaults, you will get below screen,

You can see same here in this video: How To Set Default Value For A Field in Odoo

Thanks