The easiest way (it may vary by the Photoshop (PS) version that you are using):
Open the file that you want to change in PS.
Click on Lasso Tool.
Mark the head you want to delete by using Lasso Tool. You have to make a complete circle. You will see 'marching ants' encircling the area you have selected. To make it easier, you can zoom-in the picture by using View tool.
The click 'Edit' and select 'clear'. The head you selected would be deleted.
Open the file with the new head that you want to use.
Select the head by using Lasso tool. Then click 'Edit' and 'Copy'.
Click back on the first file. Click 'Edit' and 'Paste'. You will see the new head pasted somewhere on the picture. It would show as a new layer. Move it to appropriate position by using 'Move' tool.
If the new head is smaller than the one you deleted, then you can increase it's size by clicing 'Image" and then 'Image Size' You can also use lasso tool to select an area and use 'filer' tool to change the affects in that area.
Just remember: anything that you paste on an image, it would be a 'new layer' (eg. 'layer 1', 'layer 2', etc'). The original image will show as 'Background'. You have to select that layer that you want to change.
If you don't like the changes that you made, just select 'edit' and select 'step backward' (so you can tinker and undo it).
Once you are done, click on 'layer' menu. Select 'flatten image' and all layers would me merged.
The new image would be photoshop file. Click 'File' and 'save as' and chose 'jpeg' format to save it as a regular picture.