How can I safely shrink my Windows partitions and expand Ubuntu?

The usual process is: back up your files, shrink the Windows partition in Windows Disk Management, boot a live Ubuntu USB, open GParted, and resize the Linux partition. Moving the beginning of an ext4 partition can require copying a large portion of the filesystem, so it may take hours. Keep the computer plugged in, avoid interrupting the operation, and ideally use reliable power. Check the partition layout first because the unallocated space must be next to the partition you want to enlarge; otherwise you may have to move an intervening partition or create a separate Linux partition instead.

Do not delete the NTFS partitions unless you intend to remove Windows and all data on them. Also, resizing is not a substitute for a backup: a power failure or mistake while moving partitions can make a filesystem unusable. A separate backup of important files is strongly recommended before changing the layout.

Back up anything important first. Use Windows Disk Management to shrink the Windows C: partition, since Windows can account for files it refuses to move while running. If you also want to shrink the shared NTFS partition, make sure it is unmounted and not being used by Windows, then shrink it from a suitable partitioning tool. Disable Windows Fast Startup and fully shut Windows down beforehand. Afterward, boot an Ubuntu live USB and use GParted to expand the ext4 partition into adjacent unallocated space. You generally cannot resize the active Ubuntu root partition safely while booted from it. If the free space is not directly next to the ext4 partition, GParted may need to move partitions or their data, which can take a long time and carries extra risk.