//The fastest option would be to put this into the terminal:

gsettings reset org.gnome.shell app-picker-layout

// Logout and Login to see the changes or, if you're in a hurry, restart the window manager by pressing Alt+F2, then enter r into the command input (and press Enter)

// Alternatively, you can use dconf-editor to control a number of aspects of Gnome. If you don’t already have it installed, you can:

sudo apt install dconf-editor