Spend practice time only focused on the change.  Don't strum in time, or add anything else to this drill.

Form G major chord.  Strum once.

Form B minor chord.  Strum once.

Repeat.

Works for any and all chord changes.  If you do this for 15 minutes everyday as part of your practice session the change will get more automated over the course of a few days or weeks until it's second nature.  Takes more time for some students, less for others.  But the same thing has to happen with all guitar learners.  You have to repeat the physical motions until they become automatic & second nature.

It also might be worth considering which fingering you are using for the G major.  Which finger are you using to play the low G bass note on the E string?  Using your ring finger might make it harder to switch to a B minor barre chord at the 2nd fret.  Using your middle finger might make that chord transition a little more efficient.

Hope that helps!