Hello!
If you can, back up what you have now first, in case something goes wrong. After that, boot to a Live CD/LiveUSB and select Menu > System > Partition Drives (GPartEd).
Run GPartEd, choose the drive and partition you want to 'grow', right-click on it, and select 'Resize/Move'. A new window will open. Increase the partition size to use the unallocated space, select the broken arrow (Apply All Operations), and wait while your partition is expanded.
If I missed anything, I'm sure someone will say so...
73 DE N4RPS
Rob