Tap more additional options by tapping the More icon (three dots), then navigate to Settings > Chats > Wallpaper.
Alternatively, within a chat, tap the More icon (three dots), then select Wallpaper.
If you're in dark mode, adjust the slider to dim your current wallpaper.
Tap on Change.
Choose a wallpaper category and select the image you wish to use as your wallpaper.
To revert to WhatsApp's default wallpaper, tap Default Wallpaper.
Finally, tap Set Wallpaper to apply your chosen wallpaper. (All Images From Freepik)