Thank you for your answer.
In Gslapt, I eventually received the message that thunar-volman has to (or may) be removed (this was when checking for other available updates). This was after a couple (or maybe more - I don't remember exactly) of logout + login.
After that, I logout and login a couple of times and eventually I went back to try to update Xfce using Gslapt. This time I was able to mark it for upgrade.
For other newbie users that might be interested...
Another logout + login and I was asked if I wanted to copy some previous Xfce 4.8 settings to be used with the new Xfce 4.10 (there were another 2 options).
I accepted the copy of my old settings, but some settings resulted in a different behavior. For example, some desktop icons that were previously shown, now are not there anymore.
One specific setting changed. In Salix 13.37 Xfce, the default low panel is expanded from side to side, and the notification area is located towards the right (bottom) corner. But after the update to Xfce 4.10, the notification area was always "touching" the active apps icons towards the left side. The notification area was moving towards the center / left side, according to the number of opened applications / windows.
I changed back the setting to the original (as it should had been), so the middle area of the panel (for the active applications / windows) would expand (no matter the number of windows). With this change, the notification area was permanently back in place, over the right side.
I hope I was not too vague in my general description.
Thanks to Ralvex.