Not changing UI [closed]

In 16.04, Unity was the default Desktop environment. As you used an unsupported upgrade path from 16.04 -> 21.04 it was not changed after your upgrade. So, you're still using unity-desktop as default.

Install ubuntu-desktop if not already installed and switch the Desktop Environment to Gnome before you login.

sudo apt install ubuntu-desktop

You probably also need to change your Display manager to gdm3 (Default in 16.04 was lightdm). Run sudo dpkg-reconfigure gdm3 and chose gdm3.

However... I guess, you will notice more issues related to using an unsupported upgrade path. If that is the case, you should consider a reinstall.