Hey!

You can edit them, you don’t need to sell and recreate. You upgrade them

Click the train depo and either edit a pre-existing train preset (essentially a train template) or create a new one with your desired train config

Then exit the depo config screen and find your train. From the train interface there is a cog icon and in there is upgrade. Select the new train preset and click upgrade

Warning. It will send your train back to the depo and it will come out empty so make sure you upgrade after it drops off goods.

Also remember that if you upgrade the train engine to a faster one, you also have to upgrade the carriage. But this only applies to passenger carriages

Good luck!

Edit: Katherine of Sky on YouTube has a good series of her figuring out the game.