Tap a sticker from the home view or from the share sheet select “Save to Sticker Drop”
A list of options to edit the sticker will be shown under the sticker.
Description: Descriptions are important, they allow people using VoiceOver is understand what the sticker is. If not provided the Sticker description will just be read as “Sticker”.
Border: Enabling this will give your sticker a border.
Border Width: Sets the thickness of the border around the sticker.
Border Colour: Sets the colour of the border around the sticker.
Drop Shadow: Adds a drop shadow, lets the image standout from the background.
Corner Curl: Turns the bottom right corner of the sticker up.
Sticker Pack: The location the sticker is. You can copy or move the sticker to a different Sticker Pack if you like.
Rotate: Rotate the sticker 90 degrees.
These changes once made are instantly available in the messages app.