In Inkscape, open the XML Editor Ctrl+Shift+X.
In the XML Editor, find the <svg:text>. (You can click the text in Inkscape to jump to it in the XML Editor.)
Select the <svg:textPath> child of the <svg:text>.
On the right side of the XML Editor in the box next to the Set button, enter startOffset. Below it, enter the value you want to move along the path.
Click Set.