If I’m not wrong ,your question is how to change the kernel version in OpenWRT.

Let’s say I’m using

OpenWRT: CC 15.05
Target system: Atheros AR7xxx/AR9xxx

The current version of kernel is 3.18 and I want to downgrade to 3.17

There are 2 folders you need to care :

openwrt/target/linux/generic
openwrt/target/linux/ar71xx

Step1.Edit the Makefile in openwrt/target/linux/ar71xx/Makefile

- KERNEL_PATCHVER:=3.18
+ KERNEL_PATCHVER:=3.17

Step2.Create the configuration files for your kernel

openwrt/target/linux/generic/config-3.17
openwrt/target/linux/ar71xx/config-3.17

I suggest you just copy the config-3.18 to config-3.17. You can modify them later by

cd openwrt
make kernel_menuconfig

Step3.Try to compile it:

cd openwrt
make target/linux/clean
make target/linux/compile V=s

For the first time,the kernel source code linux-3.17.tar.xz will be downloaded into openwrt/dl. Then it’s uncompressed and compiled without any patches.

Step4. Create folder patches-3.17 then put your own kernel patches (from Candela Tech) in it.

openwrt/target/linux/ar71xx/patches-3.17

Repeat with Step3.

To make a patch file in OpenWRT ,you can take a look on this link https://wiki.openwrt.org/doc/devel/patches