|Okay, a bit of investigation and making some assumptions, I presume you mean changing the "Main Person" - that is the first person displayed when someone accesses your tree and selects "View-Tree" or "View-Family".|
From your dashboard you need to select "Change Main Person". Then go to "First Individual" and set that. Make sure you check "Use this for ALL Visitors (WARNING, see note below)". Then click on "OK".
Note, however, that every time you enter the "Change Main Person" dialogue, TP seems to toggle the 'ALL Visitors' setting, so you will need to be careful and perhaps reset it the way you want each time you enter that area.