Typically babies only start to sit unassisted ard 6-9 months so dont worry abt it! It's a gradual progress. My baby started at 5+ months but only managed to do totally unassisted and pushed himself up frm lying to sitting at 6+ months.
Do you mean on a tummy time position change to a sitting position? My girl can do this at 9mo old. At 6mo old, my girl can sit assisted for awhile , unstable. It will get better.