You have two ways. First, with the help of third-party software, such as Baidu Cloud, upload all photos to the network disk (to 1-2T), and then delete all photos on the iphone. Second, open icloud Photo Gallery and select Optimize Storage. In this way, all the original photos are automatically uploaded to the icloud cloud (automatically played slowly, without your care), and only the corresponding preview pictures are left on this machine, which will greatly save the storage space of this machine. This method may require you to upgrade your ios to the latest version.