I'm new to blender, and I wanted that the face is smooth and round like the blue line.
3 Answers
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Add an edge in the center of where the curve should be with Ctrl + R
Transform this new edge toward the inside of the object (using normal transform orientation is useful here)
Bevel with Ctrl+B with clamp overlap (press C while using bevel) enabled.
See below:
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Add a loop cut so the line you highlighted in yellow is divided in half. Move the new cut towards the middle of the blue line you drew, then bevel it to make it smooth.