If you just want to center the number, select it, and in the properties panel just activating “End position” seems to work…
If you want to re-enter them from scratch, make sure 8th notes are selected, activate the tuplet tool and enter 6:4 in the popover, enter, then change the note duration to dotted half note before you enter the notes. (or enter six 8th notes, and press t (tie) between each.) Then add the slash from the tremolo panel on the right.