- open the box and, with the Solo unpowered,
- remove the SD card; depending on the model, it may be a full size card or a microSD. 4Gb in size is what's needed, but using a 8Gb or 16Gb etc won't harm.
- You can download the image from here - extract the contents of the zip file to anywhere, and use the free Win32 Disk Imager program (en lo del Win32 Disk Imager link a https://sourceforge.net/projects/win32diskimager/) to burn it.
It is very important to insert or remove the SD card from the Solo while unpowered.
Your Solo will be back to version 1.3, but you'll be able to update to current version from the configuration pages right away.