You can flash TWRP on top of your OS installation. Most of us usually do this …just in case 
This does not remove existing data - that is as long as you are not wiping any partitions while flashing TWRP. On my phone the process is as under. You could check the process for your device
Boot into TWRP
DO NOT WIPE ANYTHING
Copy the .img file to the /sdcard/
Choose install image option - select recovery
Reboot to system
Also each time the system updates TWRP gets overwritten. So have to repeat this 