Unfortunately, if your Guild Wars account has been linked to NCSoft via accessing the ingame store, you cannot do this.
You used to be able to do this by clicking the "Edit Account" button at the bottom of the screen at the character login screen, but after they set up the store, your email login became linked to your NCSoft Account. If you've never accessed the store, it may still be possible for you to do this, so I would check there.
You can still login with an obsolete email just fine, though you will want to change your contact email in your NCSoft account options if you haven't yet. I've found the relevant Support article and linked it for you:
I am trying to change the e-mail I use to log into Guild Wars but I no longer have the option to do so. Why?
If you do not have the option to change the e-mail address you use to log into Guild Wars, this means you have integrated your Guild Wars game account into our NCsoft Account Management system by accessing the Guild Wars In-Game Store. The ability to change an e-mail address for a Guild Wars game account on a NCsoft master account is unavailable. You will need to continue accessing your Guild Wars account using the e-mail address that is currently associated with the account.
You will still be able to log into Guild Wars using the login name (e-mail address), even if that e-mail address becomes deleted, invalid, or inaccessible. This e-mail address is now only your Guild Wars login name and will not be used to send/receive any communication from Guild Wars Support. All of your Guild Wars and NCsoft master account information is now sent to the e-mail that is listed in your Account Profile within NCsoft Account Management. Please keep your Account Profile e-mail address valid and updated.
If you would like to update the e-mail address associated with your NCsoft master account, please visit the NCsoft Knowledge Base article.
NCsoft E-mail Address Change
Source - NCSoft Support Page