Thanks, that works!
let toggle = true;
function setup() {
createCanvas(100, 100);
button = createButton("one");
button.position(0, 0);
button.mousePressed(fnc);
}
function draw() {
background(220);
}
function fnc() {
if (toggle) button.html('two');
else button.html('one');
toggle =! toggle;
}