From the left navigation menu Select
Products > Import products
Next, browse or copy/paste the data from your Xls file. Uncheck save if the import was previously mapped so that you may map new headers.
Once you choose a method of upload click the upload & verify button
Example of xls file kitting multiple items to one component SKU to pull inventory. both item id's 100 & 163 will draw inventory from item id 99
Map the corresponding fields for the upload by typing in the mapping field.
After mapping is complete Click Upload & Finish.