[2019] I wish there's a way to edit the deck in the game...

I'm new on this game and I don't know how to edit my deck on the Switch / iPad Mini version. I've cleared a few missions on the Realm Trials (the first 6 ones) and won my first 2 casual games, but the next casual games after that are mostly losses since the enemy player has better cards or deck than mine, so it's unfair for me as a new player who has just started the game last week. I wish they should let me battle to those same people as me who is new to this game as well! I'm not happy with those losses to be honest! I like the game, but I wish there's an option to edit the deck so I can remove the cards that I don't like and replace them with better ones... :/