Shaye, several possible misconceptions here; from your questions, I'm assuming you're in the process of learning Lr, so forgive me if I tell you some things you already may be aware of.
First, Lr doesn't 'save' files. The adjustments you make are automatically saved in Lr's catalog database file, along with pointers/links to the actual image files which are stored where you imported them from on your hard drive. If you need access to files with the adjustments, you'll have to *export* them.
Secondly, dpi in a digital image file is fundamentally a meaningless number. Unfortunately, many people (editors, clients, designers) don't know this, and insist that images have a specified dpi 'resolution'. DPI only really matters to a program which is trying to 'print' a file using a print size specifed in inches. Lr does allow you to specify a dpi/ppi, in the export dialog, which you should be using, per paragraph 1 above.
Erase what? Lr is extremely, extremely flexible in returning to earlier steps in the adjustment process. Tell us more about what you want to do, and we can give more specific advice.
I see this is one of your first posts, if no one has said so, Welcome to the forums!