Measuring risk of source code changes?

Question

My manager asked me to write a estimation of work hours and a risk estimation of source code changes for a defined task.

While the first is no problem for me and there are many resources on the web, I cannot get my head around the latter one.

I already asked for a clearer description of risk estimation and got the answer that the risk for the need of "follow-up code changes due to the changes made" and "potential loss of stability of the overall software" should be stated.

How can I approach this task (thumb rules, documents about risk estimation, ...)?