On iCloud.com: go to Account Settings, click the picture next to your name, then drag an image file onto the picture. Or click Choose Photo, then follow the onscreen instructions.
On your Mac: choose Apple menu > System Preferences, then do one of the following:
macOS 10.15 or later: click Apple ID, click the picture above your name, then select a new picture.
macOS 10.14 or earlier: click iCloud, click the picture above your name, then select a new picture.
On your iPhone, iPad or iPod touch: go to Settings > [your name], tap the picture above your name, then follow the onscreen instructions.