Here is a formula which you can use for testing: $$x^2+y^2=z^2.$$
The MathJax contextual menu contains various items; after you bring up the menu, you can go to Math Settings → Math Renderer and then choose one of the renderers. Note that the methods mentioned below need to be applied on a correctly rendered formula, like the example at the top of this post.
Right-click on any formula using your mouse or trackpad, or tap+hold on any formula if you're on a touchscreen device.
Control-click on any formula. This operation (also called a "secondary click") generally works like right-click does on Windows. See the macOS User Guide "Right-click on Mac" for more details.
Tap+hold on any formula. (Tested on FuntouchOS 12)
In Safari on an iPhone, use double-tap+hold on any formula. In other browsers, you may instead have to use triple-tap+hold (tested on Brave Browser running on iOS 16.0):
There currently does not seem to be any way to bring up the MathJax contextual menu on an iPad, even when using an Apple Pencil. (Tested on Safari and Brave Browser running on iPadOS 15.7.)
A couple of Apple Support discussion threads regarding this: Right Click on an Ipad and Right click with pencil.