If you want to plant specific flowers, you will have to remove the grass first or it will always be a problem and make the area look weedy. It will continue to grow tall and throw seed.

The other option is to convert it to a bee lawn and you can do that simply by overseeding. This is best done in the spring or fall. 

https://extension.umn.edu/landscape-design/planting-and-maintaining-bee-lawn

https://beelab.umn.edu/bee-lawn

If you want to remove it and start over first mow it. Then, I would consider solarization - then planting what you would like to see there. 

https://extension.umn.edu/planting-and-growing-guides/solarization-occultation