How do I increase the size of the C drive?

This cannot be done safely with Windows running from the partition that will be resized.

There are two choices:

a) Make a bootable USB drive with the drive cloning software of your choice.  Shut down the system and attach the new drive.  Boot from the USB drive.  Clone to the new drive.  Shut down, remove the bootable USB drive, remove the old drive, boot to BIOS, set the BIOS to boot from the new drive.

b) Take both drives to another system and attach them to the system.  Use any Windows drive cloning software.  Clone to the new drive.  Shut down, remove both drives, remove the old drive from the original system, install the new drive, boot to BIOS, set the BIOS to boot from the new drive.

This cannot be done safely with Windows running from the partition that will be resized.

Yes it can. Launch diskmgmt.msc and then shrink the C:\ by right clicking and shrink. It's windows 10, no need to faf about with boot disks.

Regards

Alex

Oh or you can expand it, either way.

Better do it outside Windows, so use this instead:  https://www.paragon-software.com/free/pm-express/ 

And after install, create the boot medium.   

It's done safely within windows. And you have a backup, don't you?

It makes zero difference if it's done within windows or not, it's not 1999 it's 2020, Operating systems are MORE than capable of shrinking or increasing disks due to the File Allocation table that they use. I.E ExFAT, NTFS etc etc.

Agreed.

But if there is system recovery partition next to C partition, you cannot use the built-in disk management utility to do the task.

Yep, if you disk layout is non-standard, then you are right, Jackie Man.

I used MiniTool Partition Wizard.   That seemed to work fine.   I am attaching the snips before and after.

Yes, that was a non-standard layout which needed 3rd party tools.

Thanks to all contributors.