I've tried several approaches and it still boots directly to Windows 10. It is a Toshiba Satellite Radius L15W-B1208X with Windows 10 the original OS.
I read and tried to follow all prerequisite steps in this helpful information-packed answer, and first tried Ubuntu 14.14 LTS, which installed but showed no dual boot menu.
Various answers and sources such as this one have suggested I could then tell the UEFI to start with Grub 2, but I see no such option. This Toshiba's UEFI config menu has never shown anything but the hard drive and USB as boot sequence options. The only other relevant setting seems to be Safe Boot on/off, which has never seemed to have any effect on behavior at any point in the hours and hours (and more hours) of trying things I have done. There is no "boot mode" option as suggested here - I suppose it is UEFI-only (no legacy option)?
The info-packed answerer said 15.04 has always just worked for him, so although I would prefer LTS, I tried Ubuntu 15.04, telling it to replace the 14.14 installation, and it is installed, but is equally unbootable.
I saw the suggestion that Toshiba may be violating the UEFI standard and annoyingly testing for "windows" in the file name and that I could rename the Linux bootloader to windows... but I don't know how to access the EFI partition - I assume I'd need to know how to find and mount it in Linux? I didn't see complete enough directions for my familiarity level, so I didn't try that.
Following advice on the WindowsDualBoot page of the Documentation section of the Ubuntu web site, I tried using a EasyBCD and managed to wipe my BCD making Windows 10 unbootable but I actually managed to fix that thanks in part to Toshiba's recovery system.
The most recent thing I've tried is Linux Boot-Repair, which I ran in all its steps but it had no visible effect. Its docs say the final steps should be to show its output data to my favorite help source, so here I am, and here they are.