Thanks for all your advices. Issue has been resolved. I hadconnected below scenerio and was able to upgrade the MX450's to 15.44 and it has started detecting the SFP for copper.
1. I had connected internet copper cable on one of the switch(MS350) port(copper)
2. From the same switch(MS350) port 49(Fiber slot) I had ran the fiber cable to MX450 on Internet port with SFP(SX)
3. As soon as I second step it has started detecting it is SX it's own slot of Fiber.
4. I had confugured the IP address on MX intial page and it has started connecting to Dashboard and downloade the firmware.
4. Once the device is on 15.44 then i had connected back to my original scenrio and it started detected and i can the access the device in dashboard.