How do you learn to edit your novels?

You don't.  I didn't, anyway, yet I am a ferocious editor of my own work and a brilliant proofreader.  This latter quality has been discovered in my Real World job and everything that is written is brought to me;  I invariably find a hundred obvious mistakes that everyone else has missed.  It all boils down to experience.
When I first started writing (I was twelve), it was by hand.  Nothing got edited!  When my typing speed overtook my handwriting, I didn't do much more than correct mistakes with a bit of Tippex.  My first PC  (it was an Amstrad....ahem) meant I could do a weeny bit more editing, but even then, I didn't do much.  I wasn't a proper writer yet and considered that everything I wrote was perfect first time.
HAH!
I turned into a Real Writer when I began the Fleet Quintet.  Working more professionally, using what is now an ancient version of Word, I was far more willing to rip things apart;  to rewrite and rewrite until it worked;  to throw out what didn't work and begin again.  Nowadays, the REAL writing for me begins when I do the Big Edit.  My writing seems - generally - to follow a particular pattern.

• The story is created in my head, then in notes.
• The first draft is an attempt to get the story down and is usually an excruciating process of self-doubt, anxiety, wordlessness, and zero inspiration.
• This "first draft" will consist of several edits, usually rewriting of earlier bits because something later on doesn't work.  So there's still a lot of plotting going on as well as character changes, even name changes.  Sometimes there are changes from third to first person.  Sometimes I restart the whole damn thing from the beginning.  But this is ALL still part of the "first draft."  I'm still getting it down on paper, as it were (or on a computer screen).
• The Big Edit comes when the first draft is finished.  I might have a rest period before I start this.  It usually entails reading the whole story in one go (or several if it's a novel) and making notes but NOT changing anything.  That comes later.  Depending on how well the first draft went, the Big Edit could vary in length.  I've been known to abandon a story altogether and then come back to it and finish it with a flourish.  It happens.
• The Red Edit is when I print out the story and read it VERY closely indeed, armed with a red pen.  This is probably closer to proofreading, though by the time I get to the Red Edit, I've already done a thorough spell and grammar check and would  have proofread it before printing it.

With my current WIP (the Prizewinner), I'm doing a combined Big and Red edit.  Hence Big Red Edit!  When I picked up the novel again a few weeks ago, half of it had already had its "red edit" but with loads of notes.  What I really wanted was to have the notes in red on the page, not on a separate bit of paper.  Instead of redoing that first half (it's a LONG novel), I'm now Red Editing chapter by chapter:

• Read the chapter through thoroughly, correctly everything on the page with red pen that you would normally do on a computer.
• Then make these corrections on the computer.  This gives you a SECOND chance to see your corrections which means you really have to think about what you are doing.

So far, this is going quite quickly.  Sometimes a red pen edit doesn't work when you see it on the screen.  Sometimes I see something on the screen that I missed.  
In the end, I'll end up with a novel that has been thoroughly overhauled, which is what I'm aiming for.

Everything you see on these pages has actually been cut! It's the biggest, scariest cut I've ever made - a three-page scene totally dumped!