You should open your PSD file in Adobe Photoshop. Go to “\sources\psd” folder and there you will find all the PSD files for your OsCommerce Template.
You can edit any text using text tool (hotkey “T”). To add a new image open image file and then just drag and drop it to your template opened in Photoshop. When you make all the changes to your PSD file you have to save the slices that have been modified. To do it press CTRL + ALT + SHIFT + s (or go to “File–>Save For Web”) to call save for web window. Then select all slices that you want to save (hold shift for multiple selections). Press save. Select “selected sliced” and “images only” in next dialog. Choose “osc” folder and then save. If you have done everything right you will be asked whether or not you would like to replace or not replace the old image files, choose replace.
Then you will have all the updated images in “images” folder of your OsCommerce locally on your machine. Now you should copy this “images” folder to your server. Replace all the images and then they will be updated.
You can open your online shop, refresh the page and will see new images.