Rocket League

From the Main Menu, go to Garage > Manage Inventory > and then select the items you wish to trade in. Once five items are selected, pressing will trade them in. As long as they are able to be traded, and the same quality (Common, Uncommon, Rare, Very Rare, or Import), you can trade them in and get the achievement.

Note that Exotic, Black Market, and Limited items cannot be traded in. Also note that if the items are from a crate, 5 items of the same quality and the same crate have to be selected. You cannot mix-and-match items based on quality alone.

If you're like me and anticipated this achievement when they announced it, you should have already saved up items for it. It will pop right away when you trade them in, the items do not have to be from this update specifically. If you're new to the game, don't even bother yet; you won't have enough items to trade. You'll regularly unlock new items as you finish matches and unlock crates, so you'll have it soon enough.

EDITS:

wildwest08 suggests Common items might not work, so be warned. You might want to assume that they don't. I traded Uncommon items in and unlocked it, so it may be that the achievement only tracks Uncommon or higher.

NoHeroes94 confirms that items above Common value seem to work. As of now, it is safe to assume you can only trade in Uncommon or higher items.

Apparently Uncommon+ items are not easily boosted, and can only be unlocked in public matches. That means you pretty much have to suffer through public matches until you have what you need. Since drops are relatively random, estimated times are unknown.

You will at need at least 5 of one item type of rarity, if the items are from crates you must have 5 items of the same rarity from the same crate

-Go into garage
-Manage inventory

You can either just find 5 items of the same type or by pressing then going over to the quality drop down select the quality of items you would like to trade in and it will sort them for you

-Once the items you want to trade in are selected, press to accept the trade , then hit okay on the pop-up text

You will need five items of (Uncommon) grade or higher in order to complete this achievement.

1. From the main menu select Garage then Manage Inventory.
2. Select the 5 items of the grade you selected (Uncommon, Rare, Very Rare, Etc.)
3. Press to accept

Achieve Unlocked! Congratulations!

So you need 5 items of the same quality to trade up for a new item to get the achievement, to trade up simply go to the garage and go into trade in and follow the instructions.
Now the hard part is actually getting 5 items to trade if you can't find someone to give you the items, now you can get random drops from matches but that will take a very long time.
Now the easiest way I found to get items is doing the daily/weekly and seasonal challenges, if you click on them and look at them they will either reward you with xp or a little box which will contain an item and just hope you get the 5 you need as quick as possible, but bear in mind it is very random, but it will work and I think is the best way to get items, it will just take time to wait for the challenges to refresh.
Hope this helps