Agree with Christian. I just want to provide some tips on it.
You can use the CRX2RNX utility on Windows as follows:
Make sure the file isn’t zipped
Place crx2rnx.exe in the folder with the file
Open a command line in this folder
Use this command to convert the file:
crx2rnx [filename]
The *.XXo file should be created in the same directory.
RTKLib should work fine with files in .rnx format. If the file isn’t shown in the list when you try to upload it, please choose the All file format instead of RINEX OBS.