The axles on a trailer are designed to hold up a specific amount of weight, depending on the amount of load for which the trailer is rated. If you want to increase the load capacity on your trailer, you'll want to swap out the axles as well so that they don't fail under the new, heavier load. This isn't technically difficult, but does involve a lot of heavy lifting. In this case, the project vehicle is a 14-foot enclosed car trailer, but the process is similar for other trailer models as well.
Raise the trailer up in the air using the jack and secure it on jack stands. Double check that the trailer is solidly on the stands prior to crawling underneath it. Remove all of the tires using the tire iron.
Disconnect the brake lines running to the brakes, if your trailer is so equipped, using a line wrench. Then unbolt the axle from the leaf springs at the u-bolts using the 1/2-inch ratchet and socket set. If the trailer has shocks, disconnect those at this time using the ratchet.
Place your assistant on one side of the axle, with you on the other, and lift the axle over toward one side to get the axle free of the leaf spring. Repeat the process on the other side until the axle is completely free of the trailer, then slide it out from under the trailer.
Slide the replacement axle under the trailer with the help of your assistant and position it on the leaf springs by reversing the procedure in Step 3. Secure the new axle to the leaf springs using the 1/2-inch ratchet and socket, as well as the shocks if equipped. Reinstall any brake lines using the line wrench.
Reinstall the tires on the new axle using the tire iron, then lower the trailer off of the jack stands using the jack.