In the Score Editor, select the arrow tool in the toolbar (the first one on the left). Make sure the symbol object is selected in the object palette. Select a slur by double-clicking on the start of the slur. The Edit menu will appear. Choose the symbol editing operation you would like.
If you select Edit from the Edit menu, three little diamonds will appear at the left, right and center of the slur. You use these control points to shape and move the slur. Click on a control point to select it and drag it around to shape or move the slur. Alternatively you can move a control point around using the arrow keys. You can move between control points by using the TAB and the SHIFT+TAB keys as well as by clicking on them with the mouse. The middle control point is used for adjusting the shape of the slur and the outer control points for setting the start and end points of the slur. When you are finished shaping and positioning your slur, click the right mouse button or press the ENTER key to enter it into the score. If you don't want to accept the edits you've made, press the ESCAPE key.
When you are editing the slur symbol, you can automatically position the left slur point over the closest note by pressing Z. You can automatically position the right slur point over the closest note by pressing X. You can automatically center the slur by pressing V. If the slur is above the notes, the slur end points go over the closest notes when you press Z and X. If the slur is below the notes, the slur end points go under the closest notes when you press Z and X.
You can also erase or move symbols such as a slur using the other tools in the toolbar. Make sure the symbol object is selected in the object palette. Select the tool you want and apply the edit by clicking on the beginning of the slur (to erase using the eraser tool) or by dragging the slur up, down, left or right.