I tried to reproduce your issue by updating Fiddler from version 18.104.22.168 to the current one which is v4.6.20173.38786 Built: Friday, September 15, 2017 and the update was successful.
It seems that there is a problem with the Fiddler's uninst.exe on your machine. Could you please, quote the exact error message and let me know when does it appear: after you open the uninstaller or after you click on the Uninstall button or at some other point.
Could you please also elaborate on which version of Windows you are using and if you have administrative rights.
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