To round out this discussion, selecting the vertex and entering a length always changes the length of the segment that was drawn into that vertex. That is, you can only change the length by moving one end of the segment. The “from” osnap lets you do it by moving either end. Like this:

Select the polyline to enable the grips.

Click the vertex at the end of the segment you want to move which starts a Stretch command.

For “Specify stretch point or [Base point/Copy/Undo/eXit]:” , enter “from”

For “Base point:”, Click the vertex at the other end of the segment

For “Base point: <Offset>: “, enter “ext”

For “of”, hover close to the segment you want to change until an “Extension” dynamic input appears.

Without clicking the segment, just type the length you want the segment to be and enter <return>.

That will change the length and terminate the command.

The “from” osnap actually lets you move any object or vertex to a precise offset from any other location. Thanks to “MbMinx” in another post for prompting to learn about it.