windows 10 - How to rename multiple files with same name and different extensions?

I have a number of files with the same name and different extensions in a folder. Originally these files were all in .jpg format with different names.

Like:
1.j87
1.j88
1.j89

Now I've to rename each image to different name and extension individually.

I found this, but I don't know how to use this script. Can someone help?