You open cmd, type the location of the file (if its in desktop C:\Users\YOURUSERNAME\Desktop) and then type python3 convert.py (nameoffile).cht > (nameoffile).cht and its done!
Every time i try to boot up my 3ds this error appears i just changed my sd card because i needed one for switch and formated it and after i got a new one i formated it again, tranfered the files to the one that was on switch and now i cant boot 3ds