How to extend C: drive

I have my Windows 8.1 installed on the primary C: drive and I want to extend its size from other free (unallocated) space. There are lots of solutions online but none of them is 100% correct or offer assurance of not losing data. I can't afford to lose any data. How do I avoid that?

I would use the Disk Management tool built-in to Windows 8.

To access, pressing the Windows key and X, to access the Power User menu and click on Disk Management. The window below will open:

From here you can select your disks and partition them as you want. Right clicking on the disk, over the various partitions, will give you the option of shrinking or expanding the partition.

However, as Rsya Studios and Moab have stated in the comments, make sure that you back up your data before performing any changes to your partition/disk - just in case...