If your Operating System is 64 bit (x64) then you should download 64 bit version, if your computer is 32 bit (x86) then download 32 bit version.
Here's how you can find out if your computer is 64 bit or 32 bit

If you get a warning from your Anti-Virus software or Firewall while downloading the executable file, you should try downloading the Zip file version of the installer.

Uninstall instructions

How to uninstall uMark?
On Windows XP go to Control Panel , select Add or Remove Programs, find uMark in the list and click Uninstall.
On Windows Vista, 7 and 8 go to Control Panel, select Programs and Features, find uMark in the list and click Uninstall.
On a Mac, go to Applications in Finder, find uMark and drag it to Trash.