FL Studio is very easy to work with, but has few disadvantages, like not saving midi with 0 version, not saving correctly markers (didn't check yet FL 12).
FL Studio demo version giving all option active except, you can't open project, but as a midi editor you don't need to save it as project, you using export/import tools.
Cubase is very powerful DAW, but very tricky for beginners.
Ableton Live is not recommended as midi editor, is best Audio Sequencing DAW, supporting midi, but has no option import song midi files at once.
ProTools is best to editing midi and audio, is a top shelf DAW for professionals, but is not cheap.
My suggestion is try on all demo versions, you find which one is best for you.
Use YouTube to find how do things, is all there.
As changing time signature is not easy, but I can help you with basic tips.
Just tell me on which DAW you like to do it.
Nemik
Korg Kronos2-61, Korg Pa600, Novation UltraNova, Akai Professional MPK49, Akai Professional MPK225 , Alesis QX-49, APC25 Keys, Launchpad MK2, Korg nanoPad2
MOTU Midi Express XT, FCB1010 with UnO, Behringer X2222 USB, TASCAM Model 24 Mixer, Presonus AudioBox96, M-Track Plus
Fl Studio 21, Ableton Live 11 Suite.
Korg Pa5x Sounds at Performers Heaven