handling bimaps for Garmin watch face

Hello, I am new to Garmin watch faces.

I have started bulding one, and i would like to know how to handle bitmaps.

i can load the background image for my watch, and then the complications for hours, minutes and seconds.

But they are static, i want to know how to display them dynamically according to the current time.

and wether i should do that in the onUpdate() function or somwhere else.

Thank you)