If you have Windows 8 PRO, then the upgrade can be done through Windows Update. Press Windows + C, then go to Settings > Change PC Settings > Update & Recovery.
Check for update and Windows 8.1 will be installed. During the installation, the computer might restart a few times.
Secondly, press Windows + X, then go to System and click on the Get more features with a new edition of Windows. Choose Windows 8.1 and follow the instructions.