Are you lost on how to edit your contacts in that Samsung Galaxy S5 of yours? Things you may think are difficult to do often times end up becoming the easy ones. To edit your contacts in your S5 just tap on the Contact name you wanna edit from your Contacts tab and then tap again on the Contact picture with name under neath, finally tap on the pencil icon on the top right corner.
If you want to add a Photo to your Contact, just click the + icon to the bottom left of the default pic, and it will let you take an image from your Camera or from your images. There is a bug that causes adding photos to contacts to crash sometimes. Other people have reported the Contacts tab itself is crashing. Seems like a bug in Touchwhiz. This is exactly why I love rooting and flashing Custom Roms so I can be independent of Samsungs Touchwhiz interface.