What do you call it when a scam artist modifies the contents of a document to deceive other people?

Some unscrupulous scam artist decided to modify a contract that had my signature on it, to make it appear that I had agreed to things I did not. Here is a simplified example:

Bob agrees to pay Fred $20,000 for Fred’s service of building Bob a home.

The bellow signature indicates acceptances into this contract.

Bob’s Signature: Bob
Fred’s Signature: Fred

Now say Bob gets crafty and uses an image editing software, such as Photoshop, to replace "$20,000" with "$10,000".

What would this be called? What word would describe this "falsified" document? I'm not so much looking for a legally correct term, as what's legal is for the judge to decide. I'm looking for an easily understandable term. It looks like I'm going to have to explain this to a few people and I want a way to distinguish between two documents where one had been subtly altered.

Since both signatures are valid, I don’t think it’s correct to say it’s forged. I also considered fake, but not sure this is accurate as (in a sense) it is intended to be different from the original. Other words that come to mind are counterfeit and phony.

Solution 1:

To distinguish between the legitimate document and the one that was altered to be deceptive, you could refer to the falsified document as doctored.

Merriam-Webster provides the definition under doctor as a transitive verb:

2.b. to alter deceptively

The OED provides a more detailed definition:

3. fig. To treat so as to alter the appearance, flavour, or character of; to disguise, falsify, tamper with, adulterate, sophisticate, ‘cook’.

A recent example from Wired magazine uses the word to describe a video edited to be deceptive.

In July 2015, Planned Parenthood’s website was hacked shortly after the Center for Medical Progress, an anti-abortion group, released secretly recorded (and discredited) videos doctored to make it seem like Planned Parenthood sold fetal tissue.

Solution 2:

Traditionally, the word for both the faked document and the act of faking the document would be forgery. From Merriam-Webster:

2. : something forged
3. : an act of forging; especially : the crime of falsely and fraudulently making or altering a document (such as a check)

Your document has been fraudulently altered, so in your example

This contract is a forgery. Bob engaged in forgery when he secretly altered the terms written on the contract after he and Fred had both signed it.

In the particular case that you've described, assuming you are correct that "an image editing software" was used to alter the contract, this is also a particularly egregious case of photoshopping (even if Adobe's product isn't used to create the fraudulent document). Also from M-W:

photoshop
verb, often capitalized
transitive verb
: to alter (a digital image) with Photoshop software or other image-editing software especially in a way that distorts reality (as for deliberately deceptive purposes)

So Fred could say something like

This isn't what I actually agreed to; the original contract that I signed has been photoshopped.

You could also combine the two terms, for greatest clarity:

I signed a similar document, but this one is a photoshopped forgery.

_{ETA: I am not a lawyer; this answer is just about the common usage of the terms, not their legal application or implications ;-).}