Available complications vary depending on which watch face you are using. ( not all faces accommodate them ) You can either long press on your watch until the current watch face reduces in size and scroll to the right through the different faces you have on the watch - any face that is customisable will show " Customise " below it - tap on that to make changes
or
Open the Wearable app tap on Watch Faces and again scroll through the faces you have to where each face is in the centre of the phone screen and any that are customisable will show " Customise " below the face - again tap on that for the options for that face
If you don't already have any faces that will give you what you want then on the watch face page in the Wearable app scroll right down to the bottom and you will see an option " Get more watch faces - tap that and will take you to watch faces available from the Playstore - some are free and some you need to pay for
Hope this helps
