How Do I Change the File Name for Pics Imported From a PC?

I imported 300 pictures from a PC using a thumb drive, imported them directly into Photos. The pictures imported were titled when they were on the PC.
But in Photos, the mouseover shows ‘untitled” in grey. The Get Info box shows an image number only. When I try to change the title in the Get Info menu, I am unable to do so – I can highlight but the name doesn’t change.
Is there a way to change the titles for each of these pics?
Thanks in advance…
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Jools A