Click on your name at the top right and select signature. From there, you will be presented a text box to update that information. Let me know if you don't see that option.
Might it be a config issue with Xenforum ? Seems like signatures are turned off for all users, and perhaps the only reason some have them now is from the import process from the old VBulletin forum for those that had them there ? I never had one and I don't see the option to edit it under my account name and it's not contained within any of the other options
There was nothing difficult to create signature in this forum, I just gone through profile & selected the signature in the menu, i could edit my signature easily in that editor.
for changing your signature you have to click on your profile name which is shown on the right corner of your page and select the signature option from this. A edit block will shown and you can add your new signature on this it will update soon.