Tap on the Fiddler “Do not trust” certificate, then scroll down to remove it
You may need to power cycle your device to get all apps to forget about the Fiddler certificate (e.g., the Chrome browser will continue to try to use it for a while)
Go to Tools -> Fiddler Options -> HTTPS. Then click the "Actions" button and then "Reset All Certificates"
It will popup a message that it could take a while but it's really quick. Approve all popups and there you go.
Pay attention not to re-approve the certificate again (when I did it the message for approving the certificates popped up when I finished to approve all the popups.)
In Fiddler go to Tools » Options » HTTPS.
Then uncheck Decrypt HTTPS traffic and run Actions » Remove Interception Certificates.
This will remove all Fiddler certs from the Windows certificate store.
Background:
Fiddler is obviously using a kind of white hat "man in the middle" approach to decrypt and inspect any HTTPS traffic. To do that, it needs its own certs to be trusted. Therefore leaving Decrypt HTTPS traffic checked but removing the Fiddler certs as proposed in other answers does not make a lot of sense, as Fiddler can't decrypt then anyway.
Here is the procedure with Progress Telerik Fiddler Classic in its version v5.0.20211.51073.
Go to Tools > Options > HTTPS. The option to Remove Interception Certificates is greyed out, because Decrypt HTTPS traffic is still toggled ON.
Untick the box in front of Decrypt HTTPS traffic. You should be able to Remove Interception Certificates.
In the end:
Fiddler Classic's root certificate has been removed.
Fiddler-generated Certificates have been removed.
To ensure that certificates related to Fiddler have been effectively removed, in accordance with the messages displayed above, you could browse through authorized certificates with the following procedure.
Click on Open Windows Certificate Manager.
NB: if you prefer to use Windows' built-in tools, e.g. if Fiddler has been uninstalled,
Press Win+R, type certmgr.msc in the search box, then press Enter
Then:
Go to Action > Find Certificates...
In the search box for Contains:, type DO_NOT_TRUST_FiddlerRoot
In the drop-down box for Look in Field:, ensure that the option is set to Issued By. If the option were set to Issued To, you would find fewer matches.
Click on the button Find Now to list every certificate .
In my case, there was one Fiddler-related certificate left after the procedure. If that is the case for you as well, then you may want to manually delete it, by right-clicking on this entry.