Once you changed the basic disk to dynamic disk, it cannot be changed back to basic disk without deleting the partitions and rebuilding the disk.
In this way, all the data on the dynamic disk will lost. There isn't any way to convert dynamic disk directly to the basic without data loss. The most safe way should be backup or clone the dynamic disk firstly and then do a dynamic disk conversion in the Disk Management tool. How to clone or backup dynamic disk?
After you have backed up all the important data on the dynamic disk, you can convert the dynamic disk freely. How to convert dynamic disk to basic without data loss?