There are many ways to merge partitions. The safest is to use a partition manager such as Partition Magic.
However, there is ALWAYS a risk of data lose. It is best practice and a darn good idea to back up all data and critical information before attempting to adjust partition sizes.
Personally, I would consider moving your data to the unused partition. That would both free up space on the boot partition and make it a fairly easy task to back up just your files.
One last thought. Make sure this is unused drive space and not the restore partition on the hard drive.
is adjacent to the Primary Partition and the Partition is empty you should be able to merge them. I normally Clone the Primary Partition first just in case something goes wrong.
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Merging an unused partition