If you have a Yún with an older firmware: (1.5.3) before proceed you need to to upgrade the bootloader following the bootloader update procedure. If you don't upgrade the bootloader first, your Yún will become unusable.
WARNING: Following this guide will VOID the WARRANTY of your Yún
I was happily using 2 Yúns until purchasing 2 more newer units with the badly working osjs web interface. I upgraded to 1.5.3 encountering more issues.