Change Start Point of a joined tracks

If it starts at S2 instead of S1 it'll be because you joined  Route A to Route B rather than Route B to Route A.  This assumes S2 is the start of B and S1 the start of A.

Afraid I don't understand your second question.

If I have understood you correctly, for a downloaded track where you want to start in the middle there are two ways of doing this.  You can make two duplicate tracks. In one of them erase all the points up to where you want to start. Then in the second duplicate, erase all the points from the middle new start to the end. Then join the two tracks to get a new track beginning at your new start.

The other way is just to copy and paste groups of track points into a new track in the order you want to follow them. You can select sections from several tracks if you want to and use invert if you want the track points in the opposite direction before you copy them.

Routes and tracks have a direction of travel associated with them.  The result of joining them will depend on the order they listed in the joining dialogue.  They will be joined tail to head in the order they listed from top to bottom in the joining dialogue.

For example, if both track A & B start at point S1 and go to S2, then one of the tracks will have to be inverted.  If you wanted to travel S1 to S2 via track A, then track B would have to be inverted and track A would need to be list first (above) track B in the joining dialogue.

Similarly, if you wanted to start at S2 and travel S2 to S1 via track B, then track B would need to be inverted and it would appear first (above) in the joining dialogue.

As mentioned above, it is a good practice to carry out these operations on duplicated objects.  For routes, you do not need to duplicate the waypoints/via points.  But, you also do not want to move any of them in the duplicates.

Thanks to all. Yes, the problems was that I joined to "cutted" tracks (fig 1), so the original start  of the tracks was not present.. I will play with the order of joined tracks. Thanks

About my second question... let say original tour starts with a uphill and then downhill; but I want to start from top mountain and downhill first. Can I move the start point without to re-trace the track ? Maybe Cutting the track ? (fig. 2)

If you follow my post above, you can easily move the start of the track to wherever you want it to begin. You are essentially cutting the track into two sections at the new start point and re-joining them in a different order to make a new track with a new start..