Change a secret key in an embedded device
What's proposed is unsafe if used more than once: assume $\text{Sk}_1$ is (re-)established this way by sending $\text{RN}_1$; then, after compromise of $\text{Sk}_1$, $\text{Sk}_2$ is re-established this way by sending $\text{RN}_2$. The adversary can find $\text{Sk}_2$ as $\text{Sk}_1\oplus\text{RN}_1\oplus\text{RN}_2$.

Also, $\text{RN}$ does not seem integrity protected, allowing the adversary to play nasty tricks.

A better practice is to send a fresh $\text{Sk}$ protected by authenticated encryption with a dedicated Key Enciphering Key (having the role of $\text{SRN}$ here).