You can find older versions of the IDE, including version 5.1, on the official Microchip AVR® and SAM MCU Downloads Archive .
Rohan nodded in agreement and decided to verify the authenticity of the website. After some research, he found that the website was not an official Atmel (now Microchip) website, and the download link seemed suspicious.
However, I can offer a on how to obtain the correct , safe, and fully functional software for AVR development, avoiding fake or low-quality downloads.