If during install you decide to stop the installation, it is possible to reboot, but please think twice before you do this. Once a partition has been formatted or updates have started to be installed, your computer isn't in the same state anymore and rebooting it could very well leave you with an unusable system. If in spite of that you are very sure rebooting is what you want, go to a text terminal by pressing the three keys Alt Ctrl F2 at the same time. After that, press Alt Ctrl Delete simultaneously to reboot.