How can I delete an incorrect tag synonym, flag it, or vote it down?
The tag synonym in question is "https -> tls", its incorrect because HTTPS is HTTP over TLS/SSL, and it's a bit wrong to mix questions that are tagged with HTTPS, with for example questions that are about SMTPS (SMTP over TLS/SSL).
I would agree on the tag synonym if HTTPS was widely misused as a synonym for TLS/SSL, but I don't think so. It's better to fix the few questions that do have an incorrect "https" tag for a non-HTTP protocol over TLS/SSL, because autofixing all "https" tags to "tls" means a question about for example a security vulnerability or security rule only applicable for HTTP tunneled over TLS/SSL (for example CRIME or mixed-content rule) would get sorted under same questions to for example SMTP traffic over SSL, and it's not really right.