yes, you can always change it back at any time.
just checking.... do you mean DNS or IP address?
and what is it you are trying to achieve?
your DNS address is your 'gateway' to the internet (often called your gateway address) so changing it will effect things but it depends on what it is being changed to and why.
think of it as, IP addresses are for LAN's (from the modem looking inwards at your PC's) and DNS's are for WAN's (from the modem looking outwards to the world).