How does one uninstall it? (i admit, I clicked before I thinked.) If it slows down my pc too much how do i go back to the non-hr? it's my first steam dlc

You will have to reinstall the game. Now, since the Hi Res comes as BSA's, it may be possible to archive the current BSA's before you download the new ones. Then move them around. But, I don't know if there is an Archive Invalidation process. I also don't know if the new BSA's replace the current ones, or are loaded with the current ones.
You might want to "Back up local game files" before downloading, that might be the easiest way to undo this. If you don't choose to compress, it won't take too long to back up.