How do I defragment Windows paging file

A simple solution (which won't move the pagefile to the beginning of the disk) is to disable virtual memory, then reboot and finally re-enable pagefile (this time with a fixed size).

This method will ensure your new pagefile is in one "chunk" on your disk and will also prevent any future fragmentation of the pagefile.

Moving it to the beginning of the disk is relatively pointless nowadays, but for defragmenting it you can use PageDefrag. Note that fragmented files are not an issue on SSDs.