If you had Win 7 it was probably installed as Legacy (MBR).  The upgrade would have left it in that configuration. If you have a UEFI bios you will need to do a clean install and select the UEFI version of the Install Media.  The install cannot convert the current hard drive to GPT unless all the partitions have been removed. So basically, if you don't want to redo your system, leave it in the Legacy configuration.
I have an Inspiron 660. Last fall I upgraded Win 7 to Win 10/64.  I was looking at the System Information recently and I noticed that the BIOS mode was set to Legacy. I see that UEFI is a step up from BIOS. Is it safe to switch to UEFI after upgrading in Legacy BIOS mode?
Switching BIOS legacy mode to UEFI after Win10 upgrade from Win 7