Can I increase images 300ppi that are embedded in illustrator?

Hi there, I was wondering if someone could help me with this issue,

I have to prepare a file for print and some of the images are below 300ppi.

The images are embedded in illustrator and I dont have the packaged file.

When I click unembed and edit in photoshop and increase the ppi it doesn't increase the resolution in illustrator when I click on links after saving. 

Any suggestions?

My first thought was to get request a packaged illustrator file /original file and then increase ppi from those links. I also read somewhere sometimes you can't fix low resolution photos, thanks !