Change scorestreaks once I obtain it

You can press down or up on the D-Pad to cycle through your scorestreaks. Whichever one is highlighted in the scorestreak list is the one you will use next.

If you get an item via a care package, it will be in a fourth slot at the top of the list. Otherwise, they will be in the order they are unlocked.

Whichever one you've unlocked most recently is the one that will be used by default.

Scorestreaks must be unlocked and selected out at the root menu of the multiplayer interface, shown here:

Note that once you enter that menu, you will see all of the Scorestreaks available for your level - but you may still need to spend tokens to unlock Scorestreaks which you have not previously unlocked, in order to make them "selectable". This is straightforward, and there are on screen indicators of Scorestreaks that require you to spend unlock tokens.

A few other possibly relevant details:

• You accrue "points" during each round for doing basically anything - killing a foe, having a UAV up while others kill foes, shooting down a UAV, etc.
• You may only have 3 Scorestreaks "activated" at any time.
• Each Scorestreak has a required point total. Hit that total, when that Scorestreak is activated, and you'll get access to that Scorestreak for use immediately during that round.
• An onscreen indication appears when you have earned enough points for a Scorestreak. I don't play on PS3, but I suspect that like XBox you'll need to D-pad right to select that Scorestreak and then for some Scorestreaks hitting the "use" button is also necessary ("X" on XBox).
• Scorestreak points reset each time you die. If you don't get enough points, it's possible to go a whole round without getting a Scorestreak.