well most users here give a short hint "read about blink without delay()"
In my opinion this is even worse than say nothing.
Especially if the TO is already on the wrong track of
"replace delay() with millis() and you are done"
@alemartk
read this tutorial that explains the details of how non-blocking code based on millis() works
After reading this tutorial go on with this state-machine tutorial
I'm pretty sure that you still will have detail questions how to adapt the examples to your code
just ask all your questions