You're all good in wanting to tweak the default journal for your Odoo invoices. Picking the wrong one can really mess things up! Luckily, it's a pretty straightforward fix through the interface. Odoo usually picks the default journal based on company settings, but it might not match your needs. So, what you gotta do is head to the accounting module. From there, hit up the 'configuration' menu and choose 'journals'. You'll get a list of all the journals in your system. Find the one you wanna set as the default for your invoices. Make sure it's marked right, like as the ‘sales’ or ‘customer invoices’ journal.

If you're dealing with multi-company setups, make sure this journal is tied to the correct company. Odoo sometimes just goes with the first journal it finds, so putting your preferred one at the top might help too.

And hey, if the settings aren't sticking, maybe check if there are any customizations or third-party apps messin' with the defaults. Custom changes can sneak in and alter things on the sly. It can be a bit tricky, so doing a quick test after changes is a good move.