How to deactivate LoRaWAN gateway, I did not find this option only delete option is available
Turn it off! (Power it down)
Not sure what you really mean/want with this question… What are you trying to achieve…if remove from TTN then ‘delete’ is indeed the way forward…
I have a TTIG and its only really used for ‘research’ purposes.
I think I have seen one, very weak on the limit, signal from another node on it.
So to save energy, protect the environement etc, when its not in use, I ‘deactivate’ it by removeing the USB power lead, seems to work …
I use a drill - in hammer mode if the gateway has really annoyed me.
I want to deactivate my current Gateway to register with another LNS because the system doesn’t allow having multiple LNS running for the same gateway at the same time. I don’t want to delete the current Gateway because I don’t want to go through the process of configuring it from scratch again
Then it will depend on which gateway/model/firmware. Simply put if you can log into the GW and change its LoRaWAN settings - typically the new target LNS and power cycle/reset and it will then ‘talk’ to the new LNS. To revert to existing, change target server settings back. If using BasicStation and CUPS it may be a bit more involved, and depending on what PF you are using you may need to set up authorisations on new LNS, certs or whatever…as ever the devil will be in the detail and no details means no help! 
Getting up on another LNS will be out of scope for this forum so you may need to use theirs for the details… 
Which system - TTN or your gateway?
If you search the forum you’ll find discussions about the perils of running a gateway against two or more LNS and the potential for breaching local radio regulations.