how to modify value inside html tags by class name

please help, so i given this string <p><span class='math-tex'>\\( x&gt;0,y&gt;0 \\)</span></p>

and i want to modify the inside value, so it would render with math jax

to be come like this <p><span class='math-tex'>$$\\( x&gt;0,y&gt;0 \\)$$</span></p>

there are $$ wrapping the value.

i am clueless how to use math jax properly, i think this is the best way to solve from my previous thread -> (How to use math jax?)

but i have no idea how modify the those value when the window load on start