You can get the file snagit.dmg from the link above. It's a good experience to check the file for malware. So, use your antivirus program or try a free one – Bitdefender Virus Scanner.
Download the installation file, double-click to run it. The Installer will do the work for you, in another case, drag the app to the Applications folder.
You can uninstall Snagit without any problems. Click on the Applications folder and move the needed file to Trash.