Open your resource pack folder
Go to `assets/minecraft/textures/entity/shulker`
Find the shulker box texture file
Edit the texture image in an image editor
Save the edited texture with the same filename
Keep the image size and format unchanged
Make sure the resource pack has the correct `pack.mcmeta`
Place the resource pack in the Minecraft resource packs folder
Enable the resource pack in Minecraft
Test the shulker box in-game
Repeat for each shulker color if needed