Change from My Cloud Mirror 4 TB to My Cloud Ex2 Ultra 8TB - How to transfer files?

My Cloud OS 3 Personal & Network Attached Storage My Cloud EX2
1

Hello,
actually I’ve the My Cloud Mirror 4 TB 1. gen. Yersterday I bought the My Cloud Ex2 Ultra 8 TB.
What’s the best way to transfer the files?

Thanks a lot.