how can i change an avatar i already created?

[JuanitaDeCCNVLE]

How can I change an avatar I already created?

[Rolig Loon]

That all depends on how you "created" it.  If you chose a starter av from the avatar picker in your viewer, just open the picker and choose a different one.  Or open your Library >>> Clothing >>> Initial Oufits folder and drag one of those folders onto your av.  Or go shopping and buy a different one.  Read this >>> https://community.secondlife.com/t5/English-Knowledge-Base/Controlling-your-avatar-s-appearance/ta-p/700709

Or maybe you meant, "How do I change clothing on my av?"  Again, that all depends on what kind of avatar you have.  You really should read this article to get started >>> https://community.secondlife.com/t5/English-Knowledge-Base/Buying-clothing-that-fits-your-avatar/ta-p/2570096 and this one >>> https://community.secondlife.com/t5/English-Knowledge-Base/Customizing-a-classic-style-avatar/ta-p/2940719

Or maybe you really did create your av and are trying to upload an improved model.  If so, read https://community.secondlife.com/t5/English-Knowledge-Base/Uploading-a-mesh-model/ta-p/974185

Regardless of what you meant, be sure toi read https://community.secondlife.com/t5/English-Knowledge-Base/Avatar-Rendering-Complexity/ta-p/2967838 so that you understand the importance of using clothing and attachments that have a low vertex count.  If you use an av or clothing that is too complex, it will be invisible or will render imperfectly in some viewers.