Pressing F5 will bring up a menu for changing the time signature, tempo and other things. But for 6/4 time signature you will have to either change it to 3/4, that is 3/4 for two measure to match one measure of 6/4, or 6/8 and change the tempo to match the 6/4 tempo. Note that the song will stay with the assigned time signature until you change it again.

But your biggest problem would be in finding two styles that match. Many 4/4 styles do not sound good in 3/4 or 6/8. I am talking about RTs as MIDI can be edited very easily to match them.

Personally I think you might want to start with a simpler song to learn BiaB. Things like changing styles, tempos etc in a single time signature can be a very valuable learning experience. As you just found out BiaB does not do all time signatures and you may find out that it also doesn't do all the chords you want either.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware