What is Tampering in STRIDE and what security property does it violate?

Modifying data or code without authorization — it violates Integrity.

Tampering:

Real-world attacks:

• Tampering with a shared library - attackers who breach a widely-included script (e.g. an analytics provider's JavaScript) can alter what it does on every site that loads it — such as silently rewriting a crypto-withdrawal page to send funds to the attacker.
• Smart-lock tampering - manipulating the firmware or signals of an internet-connected door lock to unlock it.
• Man-in-the-middle attacks - sitting between two parties and modifying data in transit.

Mitigation: Digital signatures, checksums, secure channels, code signing.