It is possible that Stable branch will skip Plasma 6.4 entirely and go straight to Plasma 6.5, as the next Stable snapshot will probably be the next Manjaro release (25.1.0 a.k.a. “Anh-Linh”). This was mentioned a while ago when the same question about 6.4 was asked in relation to Testing branch:
Today’s Testing update contains this advice:
Testing branch has now been on Plasma 6.4 for nearly a month. You could give it a try by switching to Testing branch - the process is simple to do. As everything has already been tested by Unstable branch, Testing branch is very stable. I consider it to be the best of the 3 branches - more stable than Unstable, and more up-to-date than Stable.
To switch to Testing branch:
sudo pacman-mirrors --api --set-branch testing
or, to switch to Unstable branch:
sudo pacman-mirrors --api --set-branch unstable
After you changed the branch, rebuild the mirrorlist and update your packages:
sudo pacman-mirrors --continent && sudo pacman -Syu