For instance, you can always use the Mask of the Shapeshifter to change your basic appearance. Those who purchased the Deluxe edition of Baldur’s Gate 3 will find the Mask of the Shapeshifter at their campsite in the nearby inventory chest. By equipping that item in your character’s “Helmet” inventory slot, you will gain a skill called “Shapeshift.” Click on that spell and then click on your character, and you’ll be presented with a list of optional appearances that essentially double as disguises. So, if you happen to choose to disguise yourself as another race, you’ll gain access to the unique interaction options that race would typically have access to. You’ll even gain access to some of that race’s unique bonuses, which is especially handy when you’re trying to overcome certain puzzles that require a specific touch.
Unfortunately, the Mask of the Shapeshifter doesn’t allow you to fully-customize your appearance. Furthermore, there is no known way to acquire the Mask of the Shapeshifter if you did not order the Deluxe edition of Baldur’s Gate 3. If someone discovers another way to craft or acquire this item, we’ll be sure to let you know.
You’re not completely out of options if you didn’t purchase the Deluxe edition of the game, though. Most notably, Bards, Clerics, Rogues, Rangers, Wizards, and Sorcerers can gain access to the “Disguise Self” spell, which essentially allows you to replicate the Mask of the Shapeshifter’s basic functionality. Warlocks can also access the Mask of Many Faces ability, which is their class’ equivalent of the Disguise Self spell. Every version of that spell will allow you to access the benefits of the race you disguise yourself as.
Just note that these disguises will automatically wear off after your character takes a Long Rest. Furthermore, some subclasses will not be able to access that spell at all, even if their main class’ other subclasses have access to it. For instance, Gloom Stalkers are the only Rangers that can access the Disguise Self spell, and Arcane Tricksters are the only Rogues that can utilize that ability through natural progression.
Beyond that, the only other way to notably alter your character’s appearance is to change their currently equipped items. Baldur’s Gate 3 is one of those RPGs that alter your character’s appearance in order to reflect their equipped gear, which means that each new piece of gear will change your look at least slightly. That’s probably not the expansive appearance-altering option that you’re looking for, but it’s always something to consider.
Finally, you may eventually be able to alter your character’s appearance mid-game via Baldur’s Gate 3 mods. At the moment, though, I can’t find a recommended mod that allows you to change your character’s appearance at will or through an NPC. However, I’d be willing to bet that some ambitious modder out there will find a way to add that functionality to the game.