Make active tab you want to rename, but in "Qt Object browser" should be select exactly all QTabWidget (not specific tab), then in the Property Editor (if you can't find it make sure it's visible by using the View->Property Editor menu item) scroll down to the bottom and look for the
currentTabText property. You can change the tab's name right there.
In Qt Creator Designer, if you can't seem to find the
currentTabText property, you have probably selected the tab itself in the the object tree. Make sure to select the
QTabWidget, which should happen automatically when selecting the tab in the form preview.
Also you can use this at runtime:
ui->tabWidget->setTabText(index, "New tab title");