Unless you want to learn to use the REST API and develop a script to do it, the easiest way is probably to select all of fixlets, and Export them from the console. That will create an XML file containing all of the fixlets. You can edit the XML file in a text editor and do Find/Replace on the fixlet names, save the changes, and then Import it back to the console.
This will create all-new fixlets with the new names. If the original fixlets were assigned in Baselines, those links will be lost and you’ll need to recreate/update the baseline content using the new fixlets.