Default [Q] How to flash boot.img ?

I am running Viper 5.8.0 with 4.19 based firmware & S-Off with TWRP 2.6.3.3 & had installed a custom kernel. Now i want to flash a different custom kernel. Can someone please tell me the exact steps or point me to the HTC One specific thread where it is discussed how to the boot.img of the custom rom zip through fastboot or custom recovery ? & whether it is actually necessary to flash the default boot.img while changing kernels of different developers ?
P.S. I had searched & had got results but they were quite old posts & of different devices.