Not sure your Windows operating system, but if you are using Windows 10, you can try MBR2GPT.exe. It allows you to convert MBR to GPT without data loss. You can check the detailed tutorials with video.
If you are using other operating system besides Windows 10, you give a try with aomei partition assistant(just search from google), and also here is a guide with video
But you'd better to back your system and data up before the conversion, good luck!