Shorten existing route from start

Hi,

Anyone know how to shorten a pre existing route from the start. 

For example; a 10km hike now starts 2 km further down the track than previous. So instead of re-drawing the whole route, Id just like to cut 2 km from the start then add 2km to the end.

Adding to the end is easy, but cutting from the start seems to be not possible.

Any suggestions - thanks.