booting from install usb

[Paul Scott <waterhorse@ultrasw.com>]

Greetings,

I haven't done serious system work for many years.

I let my laptop update its UEFI BIOS which of course zapped GRUB.

I am in rescue mode from the USB stick used for install and have mounted 
the root file system and done:

chroot /mydisk (where my root file system is mounted)

I am not sure where to install GRUB with grub-install

I initially installed on a Windows 10 system.

.I have:

/dev/sda1 which is empty

/dev.sda2 which is "ef EFI (FAT-12/16/32)"

/dev/****1p[1-7]  the first of which is EFI System followed by there MS 
partitions and the last three are my Linux partitions.

TIA for any help (I have been searching the web for quite a while 
without success beyond the above),

Paul