Step 1.
You need a global variable so that you can easily track your timer by using its ID
int timerID;
Step 2.
In your function where you call setInterval do like this
timerID = timer.setInterval(interval, notifyUptime);
Step 3.
In your “change interval” function delete the existing then create a new one.
timer.deleteTimer(timerID);
// set new interval
timerID = timer.setInterval(interval, notifyUptime); // now contains new interval