In Debt Yield video yield is calculated on coupon / price, but while explaining you used issuing par value. So do we have to change the par value?

Eg. For 1st time we took par value*coupon rate/par value. (Par value=1000)

2nd, after 1 year so called market value changed to 900. Now we have calculate coupon, so what would we take. Market value *coupon rate/market value OR initial par value* coupon rate /market value. Please explain