Agree (together) when, how long and where your child is allowed to use a screen. Explain that too much screen time is harmful for his/her development and determine a fixed moment (e.g. after dinner) and a location (e.g. in the living room, in the car, etc.) where he/she can use his/her screen time.
On school days, weekends and holidays you can reach other agreements. If your child uses several screens (e.g. TV, tablet, smartphone, computer, gaming console, etc.), make clear agreements for each device. Encourage your child to focus on one screen and set an example by doing the same.
Screen time is not bad by definition and in addition to relaxing can also be educational. Have some screen time together from time to time, watching movies or playing a game with mum or dad is a special experience for a child.