Yes, but not in the way you're thinking, and not without working for it. In the way you're thinking of it—being able to learn a new spell at the low, low cost of just giving up a known spell, no, you can't replace a spell that way.
This is because there is no mechanical resource spent to learn a spell except time and monetary expenses. There isn't a "slot" that you can empty and refill with another spell. You can know about as many spells as you can find and record in your spellbook—your spells known aren't in your head, they exist only as pages in your own notation in your spellbook. If you do forget a spell, the only effect is that you now know one fewer spell—you don't spontaneously know a replacement.
The only way to get new spells is the two freebies at level-up, and by discovering them as "treasure" during play. Further, the level-up freebies are only free to learn, and once learned it's on he wizard's head to make sure she doesn't lose the benefit of those free spells because they won't be replaced if they're lost.
The only way to forget a spell ("Your Spellbook", D&D Basic Rules v0.1, p. 32) is by losing your record of it in your spellbook when you don't have it prepared, and there are no provisions in the rules for easily replacing lost spells with different ones. In fact, you don't get any of them back for free—all your spells known have to be recreated through time, labour, and money by transcribing the few you had prepared when the spellbook was lost, and there is no way except rediscovery to replace spells that she didn't have prepared that day.
So yes, if burning your spellbook and then adventuring to find new spells counts as "changing the spells in the spellbook." But I'm fairly certain that trivially-true "yes" isn't what you're looking for, and the practical answer is no.
As you say, a Wizard can learn spells from spellbook they find. So the question here is whether wizardly enemies will have spellbooks on them when they are defeated.
That's entirely up to the DM. Like any piece of loot, any enemy could be carrying a spellbook for whatever reason. If the DM wants the players to find the defeated Mage's spellbook, they will. There are a multitude of reasons for why a Mage would or wouldn't be carrying their spellbook on their person - it's the DM's choice to give the players access to this (fairly valuable) piece of treasure or not.
Remember that a Wizard doesn't require their spellbook to cast their spells (unless they want to cast them as rituals), so it's perfectly reasonable for a Mage to have hidden their spellbook somewhere once they prepared their day's spells.
So if you're asking this as a DM, the answer is that it's up to you. If you're asking as a player, the answer is that it's up to the DM, but it might well be worth your while to search the lairs/houses/dens/homes/castles/towers/whatever of enemy Mages you defeat.
Best Answer
"Pay a price based on spell level"
Note: I use the term "Pay" repeatedly. In this post this simply means to deduct from their starting cash according the published Wealth By Level (WBL) chart.
The chart at the bottom of this answer is the base amount of spells a wizard gets for free. The only answers for "how many additional spells a wizard has learned at upper levels" I've found so far are to charge 150 gp for each spell. Supposedly this is to cover the 100 gp price of the page plus the sell price of the spell in a wizard's spellbook.
This is flawed on its face since it costs 100 gp per page, but each spell takes 1 page per level of the book. So a level 2 spell would cost 200 gp to add to a book. Using the intended logic this would cost 250 gp. So this is the only answer I could come up with.
However, the biggest drawback to this system is that by upper levels the WBL table has them starting with so much coin they could effectively have nearly any spell. (level 15 starts with 200,000 GP) They would have to have additional spellbooks to hold over the 100 page limit but big deal at this point.
The other drawback is that it is ridiculous to assume that anyone has killed this many wizards or convinced that many wizards to share their books. The rulebooks make this sound like a rarity due to how wizards safeguard their research..and the normal charge is 50 gp × spell level (+the cost of writting the spell into their own spellbook). A way around this is to have them pay full price for scrolls. While this solves the "found too many wizard books" problem, it may make the price go too far in the other direction.
Ways to overcome the drawbacks:
Base Wizard Spells per Level