Can't keep meteor on v2.3, it keeps trying to update

Recently updated and found out by having some issues deploying an app, a node module no longer working past node v16.x so trying to downgrade to fix it, currently installed v2.3 via npm install -g meteor@2.3 and that worked, I have that version, but running meteor build or meteor to run in dev mode, all try to update to v2.6 how do I prevent that and keep it at 2.3?

Meteor is still on Node 14, if you use meteor npm i then you should get the package appropriate to for the Node version. As such there is no reason to downgrade to earlier versions of Meteor.
What you have described is your global installation of Meteor. What you are dealing with is the app version. Since it is trying to update my guess is that it is trying to be at the version it has been defined at. You can force a version for your app, by running the following in your app directory: meteor update --release 2.3 (replace the 2.3 with the desired Meteor version).