Sorry, it was not possible by touchscreen (of course) but by tipping the light below the less than symbol (below "computer" option). So, I'm happy 
After using my Dell XPS One 2710 (Windows10) as a second screen for my Macbook (with HDMI cable) for one session I want to switch back my Dell to its original mode (booting like a Windows PC). Instead of that I get only the message "No video detected on HDMI input". On the right down corner of my PC I can tap on the third light and it seems I can choose for Computer or HDMI (is blue/selected). But using the touchscreen I cannot select computer/does not react.  Please, what am I doing wrong/how cn I switch back to the normal PC function?
XPS One 2710, switch back from second screen function?