You look at the router setup screens which you reach via your internet browser.
In wireless security section either note down the WEP passphrase or (preferably) copy and paste the HEX code.
Alternatively you can create a new passphrase and apply that to all connected computers.
For how to access setup screens, see the literature which came with the router or download the manual from the maker's site.
WEP is not the best by the way, WPA is simpler and more secure.