You can open developer options by going to Settings > About device, then tapping on the build number 7 times until it says "You are now a developer". The developer options menu should appear on the left.
Just wondering, what are you going to change in developer options? Messing with certain ones might make the device perform and act weirdly. I would suggest trying a cache partition wipe to fix your problem first. Here's how to do one: https://support.t-mobile.com/docs/DOC-14245