this should be a job you can do yourself; new chains usually have a special link that means you can take the chain apart without having to push out a rivet. removing the old one should also be a breeze if you have a bike multi-tool with a chain-rivet extraction device.
1) Push out a rivet on the old chain so you can remove it from the bike
2) Measure the length of the old chain, and remove any extra length from the new one so that they are matching lengths. Too long and you will get chain drops.
3) Clean the chainring and sprockets at the rear - a good time to do this when the chain is off - use something like muck-off
4) Re-lube anything you cleaned
5) Put the new chain on the bike, thread it round the derailleur and join ends with the split link it came with.
Make sure you get 7-speed chain - chains come in different widths so they will fit appropriate rear cassettes - if you get single speed chain it will be too wide, and 9 or 10 speed will likely be too narrow.