This is a very common process, in a few words/steps (but you should be searching for the details in internet).
- Login from a live USB and open KDE Partition Manager or Gparted, if available.
- Unmount both windows and Garuda partitions.
- Reduce the windows partitions and assign the relevant unallocated space to the Garuda partition and confirm.
- Login to Garuda and:
- Resize/extend the btrfs filesystem with
sudo btrfs filesystem resize max / - Perform a full btrfs balance with
sudo btrfs filesystem balance start /