1, the first choice is to open a line drawing you want to modify in the PS edit box. Here, take this picture as an example.

2. Then click the pen tool in the toolbar on the left of PS with the mouse, and then stroke along the lines in the figure, as shown in the figure below, the pen path has appeared.

3. Then click on the strokes in the toolbar at the top of PS, and a color selection box will appear. Click the black you want directly with the mouse, as shown below. The lines in the following figure have turned black.

4. Then continue to click the stroke option in the toolbar at the top of PS, and a selection box will appear, where you can select the line thickness and style you want. Click the Settings button in the upper right corner of the selection box to select more options.

5. After modifying the picture, you can click the toolbar at the top of PS and select the "File" option, and a selection box will pop up. Select Save to export the picture.