Change the extension of many files

Finder’s Rename can replace “.jpeg” with “.jpg” across a selection. In Terminal, a short loop renames every file with one extension to another and never overwrites.

Without Terminal

Select the files in Finder, right-click → Rename…
Choose Replace Text: find “.jpeg”, replace with “.jpg”.

In one command

Rename every .jpeg in the folder to .jpg:

for f in *.jpeg; do mv -n "$f" "${f%.jpeg}.jpg"; done

Changing the extension does not convert the file — a .png renamed .jpg is still a PNG. To convert, see the PNG and HEIC guides.

Or just ask Titan

Rename all the .jpeg files in Downloads to .jpg. Do it with Titan

Titan is an AI that lives on your computer: tell it what you need and it does it while you watch — you press Return, nothing runs by itself. What is Titan?

Titan isn’t on this computer yet. Get Titan, then press the button again — the task will be waiting, typed in.

Related

Rename many files at once Convert PNG images to JPG