This will explain why it's important to have backups, and how to do it.
Why Do I Need Backups?
Because Second Life might decided to break or lose things. SL is very volatile.
How to Create Backups
You can't backup things you're currently wearing, and you can't copy a folder, just the contents of the folder. (This needs images to show how it's done.)
Step 1) Create a new folder. Step 2) Making sure you're not wearing any objects in the folder (skin and clothing is fine) click on the top item, then shift-click on the bottom item. It should highlight them all. Step 3) Right click on the selection and choose "open" from the menu that pops up. Step 4) Right click on the empty directory you created and select "paste." Step 5) Patiently wait for the new directory to fill up.
If some items are missing from the folder you copied into, try logging out and back in. Your inventory will sometimes not properly display copied items until you relog. In my experience, it has never failed to copy them, just failed to display them, and therefore not let you use them.
How to Restore a Backup
If you've broken your avatar or other item and you want to restore the original, delete the broken one, and do not wear/use it from the backup folder! That has the potential of corrupting your backup. You have to copy it out of the backup so you still HAVE a backup.
If All Goes Wrong
If you lose your item and your backup, the vendor should have a record of everyone that's bought their product. It can be a hassle for them, but they can give you a new copy of it. Some places have this automated, either through an item they have given you, or something you can click on at their store.
When contacting the vendor about this, it's best to copy/paste the information in your transaction history that shows that you purchased it. They can then more easily compare this against their transaction history and speed along the process.