DB2 LUW V10.5 on Windows. How to Manually rotate the Diaglog using scripts
The easiest way to rotate the db2diag.log file is to use the db2diag utility with the -A ("archive") option.  It will rename the current db2diag.log file (appending the current timestamp to the file name).

db2diag is also quite useful for searching through huge db2diag.log files (instead of trying to open them in a text editor).  It has many options to filter the file based on time, content of messages, etc.  You can read more in the documentation.