I modified some files in the Linux kernel
include/linux/tcp.h
include/net/tcp.h
net/ipv4/sysctl_net_ipv4.c
net/ipv4/tcp_output.c
Following
As you are changing some linux header, do a make first to see whether there are any side effects. The kernel might not compile. This is absolutely necessary.
I think make is also necessary for the changes to take effect for the kernel headers so that they can be exported to user space.
Just doing make modules_install will install already built modules, the modules won't be recompiled against the changed headers. Similarly for other install commands.