It's not possible to use any settings within the phone to roll back firmware however there are possibilities to roll back using other methods.
A Google Search will show you how to do it along with the pros and cons of doing so.
Anyone attempting this do so at their own discretion.
As for the icons if you don't like them then there's always icon packs in the Samsung Theme Store.
May I ask what else your not liking with the Samsung One Ui and when you downloaded it and if it's the Beta Build or the build from your phones software update section.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.