When I installed Android Studio in my Windows 10 pc, then icon for Android Studio used to be this :
But after a few days, icon has changed itself into this :
How to fix this?
In your taskbar, right click -> Properties -> Untick Use small taskbar icons. (With similar meaning, I don't know the exact original text in English).
The icon file of Android Studio, $ANDROID_STUDIO/bin/studio.ico, is an icon file containing 6 pictures, using the size of visible area to decide which one to use. Asking the taskbar NOT to use small icon will solve your issue.