AppImage: Get it from Release page, or:

1. Download the latest AppImage (remember to move the file to where ever you want to store it):

curl -O -J -L $(curl -s https://api.github.com/repos/flameshot-org/flameshot/releases/latest \
             | grep -Po 'https://github.com/flameshot-org/flameshot/releases/download/[^}]*\.AppImage' \
             | uniq)

2. Delete older versions of Flameshot AppImage

3. Set it to executable:

chmod +x Flameshot-*-x86_64.AppImage

4. Run the software:

./Flameshot-*-x86_64.AppImage

Snap:

1. Install Snap from file snap install --dangerous flameshot-app_v0.6.0_amd64.snap

2. Install multiple versions of Snaps from file snap install --dangerous --name flameshot_beta flameshot-app_v0.6.0_amd64.snap

3. See all Snaps installed: snap list

4. Run Snaps: snap run flameshot

Flatpak: waiting...