Mount a FAT32 formatted SD card on the Xubuntu desktop and copy the required files onto it, then unmount the card so that it is ready for use on the Zedboard.
steve@Linux-Steve:/home/steve/projects/petalinux/avnet-digilent-zedboard-2020.2$ cp /tftpboot/{BOOT.BIN,boot.scr,image.ub} /media/steve/petalinux