As said above if there are no indents then its a snap on back. Look carefully around the edge there will either be a little indent in the case or a little tab on the actual back (or both) insert a knife in here and use it to pop the back off. Try to use a motion as if you are slicing the back off rather than lever it, if you distort the back at all it will not go on again. On better quality watches like seiko the back can be VERY tight and difficult to get off and even more difficult to put back on. If you do manage to get it off the button cell is usually a 362 in a SQ watch the new cell will cost around £1 -DO NOT touch the new battery with your fingers it will discharge it. There is a sprung bridge to make contact with the positive side of the battery you will need a fine screwdriver to back off the TINY (extremely easy to lose) screw. All in all a fiddly job during which it is extremely easy to damage the watch, damaging the coil when you are taking the back off, losing tiny little bits from inside, cracking the glass when re-fitting the back etc. etc. I have worked in the jewellery business for 23 years and have probably fitted thousands of watch batteries my advice would be get someone who knows what theyre doing to do it. We charge £5, if you are quoted more go somewhere else.
Mrs Mac does it for the price of the battery (£4.50.)
£ 4.50 is a good price for changing a battery but if that is what she pays for them then Mrs Mac needs to change her supplier - those cells are about 70p each (quality ones, cheap ones are even less)