The same happened to me when I was using Flyme 5.1.11.0G. It introduced some severe overheating issues. Even cooling down the CPU (upper left side of phone) with Clean Master did not work. But when I updated to 5.1.12.0G, the issues were, let's say fixed.  They were not completely fixed, but they were minimized.  e.g.: from 100% to 50-60%

Method 1:
Go to Official Flyme Site (flymeos.com). Then go to "Download" page, select your device and then, choose the version you want and click the "Download" button. After download, connect your phone to the computer via the USB cable, then copy the "update.zip" file to the main directory of your device. (WARNING: Do not rename the file, leave it as "update.zip") After that, disconnect your phone from the computer, go to Files app, find the update file tap on it. A message box will appear saying "Reboot is needed to upgrade system". DO NOT CHECK THE WIPE DATA OPTION because this will delete all of your personal data (contacts, music, photos etc.) Then select the "Upgrade" button. (may be different on your device, it's the button with blue letters) Then, let it complete. After upgrading, your device will be running Flyme 5.1.12.0G! Also, a notification will be displayed saying "Successfully updated to Flyme 5.1.12.0G".

Method 2:
Go to the Upgrade app. It will check for new updates. If Flyme 5.1.12.0 G is vailable for your device, it will ask you to download the update. Select The "Download" buton, let the update download, then when it asks you, reboot and let it complete. After upgrading, it will do the same as Method 1. If the update doesn't show up on the Upgrade app, follow Method 1.

(Note: Which device do you have?)

If you have G(Global), yes. But, if you have A(Chinese), no.