where I can find health monitor File in the server in F5 load balancer

The file can be stored pretty much anywhere. What kind of monitor (HTTP, TCP ?) are you using ? If you can provide the configuration, it will be easier to answer.

why do you need to rename the file? the big-ip doesn't work like that.

are you sending a string to a server? you will have to find the correct health monitor in the list here: Local Traffic ›› Monitors

if you know which virtual server or pool this relates to you can also check there first to determine the health monitor used.