How to change my IMEI number?

BEFORE ANSWERING, READ.
I have an iPhone 5 that's locked to O2 and I have a factory unlocked Samsung Galaxy Ace 2.
For what I know, the IMEI determines it being locked or unlocked carrier-wise. I'd like to change my iPhone's IMEI for my Ace 2's IMEI, only to have it factory unlock. I'm going on holidays and I need a SIM from the country I'm going to but since my iPhone is locked I wouldn't be able to put one in, so I want to change the IMEI so that I can. After that, I'll be back to the original SIM card and carrier.
I'm aware that changing an IMEI is illegal but what I'm planning on doing is swapping IMEI, both being my OWN private phones, aka private property.
No, I'm not able to use my Samsung because it has been through it's due life experiences and is now quite broken. I can still read the IMEI luckily, not the rest of the screen though.
That all said, how can I change the IMEI on my iPhone 5?

Added (1). Yes, Lara, you can. There are programs for it. What I'm asking for is HOW to do so.

You can't.

Y O U C A N N O T, you can desire to walk on mars but that doesn't make it possible in any reality we really live in. In the real world we don't always get everything we want, like your iPhone want made by Apple, you can't change that fact, you can't decide that your phone was made by Samsung because it suits you.

that being said, the "iemi" is your unique id of that phone, the first digits identify the phone carrier, yes. Like a cars VIN, it is how a phone is tracked and controlled by the mobile phone carriers.