Go to Settings -> Installed Apps -> Downloaded -> Telegram -> Permission manager -> Display pop-up window
Or
Security -> Permissions -> Permissions -> Telegram -> Display pop-up window
Both are the same, so do either one and you're good to go
Navigate to:
/data/system/theme_magic/customized_icons/
And replace the icons you don't like
Restart once you're done
Keep en mind that every time you update an app the icon will revert to stock, so you'll have to redo the process
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