Q: how do I backup my photo in icloud ?

If you want to back up the iCloud photo library you will need to do it from a computer.

how do I backup my photo in iCloud ?

Please clarify.  Do you want to backup an iCloud Photo Library, or do you want to backup photos in iCloud?

iCloud Photo Library is meant for storing your photos in a central location and syncing the photo library between all devices, but it is no backup.

If you want to backup the documents and data stored i iCloud, you have to do it from a computer as Winston Churchill explained,  See this page on how to backup your iCloud data:  Archive or make copies of your iCloud data - Apple Support

Scroll down to the section Photos & Videos.