Actually, there is no way to change it unless you know the pattern. Lock the tablet and then input as many patterns as you can until the tablet offers you the optionto change pattern lock based on your Gmail ID and password. When it happens, input the Gmail ID and password that's already attached to the tablet, to verify identity. After this procedure you will have the option to replace/disable the pattern lock without hard-resetting the tablet.
A hard reset will wipe everything from the tablet including music, pictures, apps, settings, etc.