The steps are described in VMware KB 2139574 ("Resetting Changed Block Tracking for VMware vSphere virtual machines"):

To disable CBT:

Warning: Prior to applying this procedure, enusre the virtual machine is not running on snapshots. For more information, see:

• Consolidating snapshots in "vSphere 5.x/6.0" (VMware KB 2003638)
• Determerning if there are leftover delta files or snapshots that VMware vSphere or Infrastructure Client cannot detect (VMware KB 1005049)

1. Shut down the virtual machine.
   
2. Remove any existing virtual machine snapshots.
   
3. Right click the VM, click "Edit settings", find the "Options" tab and click "Configuration Parameters"
   
4. Set the "ctkEnabled" value to false
   
5. Set the "scsi0:x.ctkEnabled" value to false for each disk of the VM in question
   
6. Remove or rename any files ending with the *-ctk.vmdk file extension in the virtual machine directory.
   
7. Power on the virtual machine.
   
8. Power off the VM again.
   

Then enable back CBT:

1. Right click the VM, click "Edit settings", find the "Options" tab and click "Configuration Parameters"
   
2. Set the "ctkEnabled" value to true
   
3. Set the "scsi0:x.ctkEnabled" value to true for each disk of the VM in question
   
4. In the home directory of the virtual machine, verify that each hard disk contains a vmname-ctk.vmdk file.
   
5. Power on the VM
   
6. Run the backup