The following procedure will help you extend a VMFS volume:

Log in to the vCenter Server using the vSphere web client and use the key combination Ctrl + Alt + 4 to switch to the storage view. Right-click on the desired datastore and click Increase Datastore Capacity...:

On the Increase Datastore Capacity wizard screen, select the LUN that will be used as the extent and click Next to continue:
On the Specify Configuration screen, you can either choose to use up all the available free space or use the slider to adjust the amount of free space to up:
On the Ready to Complete screen, review the settings and click Finish to perform the extend operation.
The Recent Tasks pane, will show an Extend datastore operation complete successfully.