The default source arrow type in yEd is "no arrow". If the edges you create by dragging the mouse from one node to another node do have a source arrow, you already changed yEd's default settings.
To set the style for edges created by dragging the mouse from one node to another node, go to yEd's palette and double-click the desired edge style. Once you do that, the double-clicked style template's background should turn from white to blue to mark it as "default style"
If you need a "default style" that is different from all the style templates in the palette, you can
- create a user-defined palette section,
- create a new edge in yEd's editor area,
- style the new edge to your liking,
- right-click the new edge to open its context menu and choose "Add to Palette" to add a corresponding style template to the user-defined palette section created in step 1, and finally
- go to the user-defined palette section created in step 1 and double-click the new style template to set it as default style.