I built rt kernel incorrectly, so it did not boot at all and I left the old one with a different name.
I want to know how to change these files back so it can be booted??I mean by a program for SD card or something similar.
All the boot files are on a FAT32 sector, which is readable and writeable by all common operating systems.
You can place the SD card in an SD card reader, delete the bad kernel, and rename the good kernel from any Windows, or Mac, or Linux machine.