How to detach lines that are connected with end points? There is no relationship like in Solid Edge that you can delete.

It can be achieved in 2 ways actually. In Tools-Sketch Settings are options

*) No Solve Move

*) Detach Segment

Both can be accessed from command search too

that both allow dragging one line away without solving another. You cannot drag it from endpoint, you have to move the whole line.

Then reattach the other end.

Considering possible alternatives I find those several actions good balance with keeping the design intent.

In example above it would be faster to delete and recreate but surely there are cases where more design intent is involved and you can save some mouseclicks with No Solve Move or Detach Segment