To change the speed, in Arduino click Tools > CPU Speed.
But for WS2812B LEDs, the speed should not matter. The software adapts to whatever speed you choose, because WS2812 always requires 800 kHz. Very slow speeds, like 24 MHz and lower, may have problems. All the higher speeds should work.
There are 2 high performance WS2812 libraries for Teensy: OctoWS2811 and WS2812Serial. Look for their examples in Arduino's File > Examples menu. For a single long strip, WS2812Serial is best. Only certain pins are supported by WS2812Serial, so please pay attention to the comments in the code which explain the pins you can use. Both of these libraries are non-blocking, meaning your code gets to run while the LEDs are still updating. This is a huge advantage for fast animation or video, so you can get the next frame ready while the prior is still updating the LEDs. It also allows other libraries to work while the LEDs update.
Some WS2812 strips are able to accept the 3.3V signal from Teensy 3.x. Others (usually older ones) require 5 volt signal. If you have that older type, a 3.3V to 5V level shifter is needed. 74HCT245 and 74AHCT125 are the best. DO NOT use the bidirectional shifters which have 1 mosfet transistor and 2 resistors. Those are terrible for WS2812 signals (and anything high speed). Some websites like Adafruit & Sparkfun have suggestions to use those cheap mosfet bidirectional shifter for almost any application. That advice is wrong. They do not work well for WS2812 or any other application requiring high speed data.
If you use OctoWS2811, this board is recommended. It gives you the 3.3V to 5V level shifting and connectors for network cable, which is the best type of (economical) wire for the LED data.
https://www.pjrc.com/store/octo28_adaptor.html