To enable 64-bit mode, access the console and press CTRL+ALT+T. Next, select Boot options followed by Select system architecture. From there, you can select 64bit mode with either single or multi-processor support.
If you would like to boot the 64-bit mode only once, you need to enter boot menu by pressing ESC or ENTER when the GRUB greeting line appears on the screen during bootup. From the menu, you need to select the appropriate architecture. Please note the system will revert to the previous architecture after restart.