Open a terminal in the folder where it was downloaded
wget https://download.kde.org/stable/krita/4.2.9/krita-4.2.9-x86_64.appimage
mv krita-4.2.9-x86_64.appimage krita.Appimage
sudo rm /opt/App/krita.Appimage
sudo su
mv krita.Appimage /opt/App
cd /opt/App
chmod 777 krita.Appimage