Exact process will depend on which Linux install wrote the bootloader, and what that install sees as
Win10, LinuxA, LinuxB, assume you are keeping LinuxA and it is LinuxA's GRUB that is "in charge" at the moment.
Boot LinuxA, ensure that nothing from LinuxB is mounted, and use
gparted, etc. to simply delete the partition(s) for LinuxB. Once they are deleted, make the bootloader (GRUB) aware of the changes by running
Where it gets tricky is if you want to keep LinuxB but LinuxA has the boot loader or vice versa.
In that case, you need to boot the Linux you are keeping, get GRUB set up and configured, and install GRUB to the MBR. Then reboot, go back into that Linux install, and remove the other Linux install. Run the
update-grub and you should be good to go.