Check out this post for a guide about How to change your Pokemon nature in Pokemon Sword and Shield.
You can change the Nature of your Pokemon by using a mint type item. Now there is one place in Galar region that sells these mint items, and that is the Battle Tower. Now, the only way you can go to Battle Tower is that you have to beat Leon in the final battle of the Pokemon Sword / Shield Main Story. Once you have done that, you will have access to the Battle Tower.
The Battle Tower is located on the furthest building on the top of the map. Inside the Battle Tower, go to the lady NPC on the leftmost side:
Talk to her and she will sell you the Mint items.
Unfortunately, you can not buy these Mint items with normal money, you can only buy them with BP (Battle Points). To get Battle Points, you will need to beat trainers in Battle Tower, and you will only get a few Battle Points per 10 level. It’s a bit tedious, but that’s what you need to do if you want to change your Pokemon Nature using Mints Items.
Good Luck and enjoy the game!