Follow these below steps to extend your primary partition.
1. Right click on the partition (EX: C:\ ) you want to Extend into the now unallocated partition and click on Extend Volume.
2. Click on Next for the Welcome to the Extend Volume Wizard window
3. Select the amount of space in MB you want to use from the unallocated partition to use to Extend this partition
4. If you want to make one partition again, then select all of the available space for that one disk. If there were other drives with free unallocated space, they would be shown under the Available selection.
Note:It is advised that you do not extend a volume on one disk with free space from another disk. If one of the drives has a hardware failure, then all the data on that partition (volume) will be deleted.
5. Click on Next.
6. Click on Finish in the Completing Extend window
You will now see the Disk Management console with the new extended partition volume ready to be used.
7. Close Computer Management.
8. Click on Computer in the Start Menu and see your new Extened partition volume.