Improve Text Resolution?

Using PhotoShop, I'm trying to improve text resolution of a 6X4 inch
jpeg image with current resolution of 300 ppi. I need to get the exact
same size image (6X4 print) at higher text resolution.

I managed to crop the original and paste it into a new image with
higher resolution (600 ppi) and then type the text into the new file.
But I can't seem to move the text characters to the proper location
without getting errors of "The layer is locked" ect.

What's the best way to re-type new high resolution text characters
into a lower resolution image, maintaining the original size, to
produce a higher resolution result?

Thanks,

-Bill