Technically, GE would want you to make a new timer with a different interval.
However you can work around this by putting the Timer's function in the script editor. There's obviously an action for creating the timer, make that action a script editor action, and in the bottom you'll see a variables/functions captioned button, click that, and look up the Create Timer function. The good old friendly winow appears, enter the apropriate values, then it creates a line of code. Change the last number in the brackets to the right number of miliseconds. Then click ok, immediate action, as usually, and that's it.