How to change dj sona form?

If you mean DJ Sona (League of Legends) — changing forms in-game: DJ Sona has multiple musical/visual forms that players can switch between during a match. To change forms: press the ability key assigned to transform (usually E for Sona’s ultimate interactions) or use the mouse wheel while in a live game depending on the skin behavior. The exact control may vary with patches, so check the in-game keybindings and champion ability descriptions. Note: If you’re asking about cosmetic loadouts or emotes in the client, open the champion screen and look for skin/sound options before queuing for a match.

If you meant changing a skin or model file (modding): Modding game assets can violate the game's terms of service. I don’t recommend or provide instructions for modding or replacing official game files. Use only Riot-approved skins and in-game purchases to avoid account penalties.