Hi. Do you know the model number of that Garmin watch?
Usually with Garmin you need a model number and then fill a repair form.http://www8.garmin.com/support/outofwarranty.html
You might also be able to get it replaced from the place you bought it from.
Alternatively, there is a lot of replacement band available online that are compatible with many Garmin watches (again always nice to know the model) and a bunch of youtube video detailing how to change those.