Haven't messed with Proboards to much, but I did take a look.
Looks like all you will need to do is go to
ACP > Themes > Layout Templates > Forum Wrapper
Find :
<p class="sidebar__description">Short site description. Kale chips skateboard fingerstache, distillery paleo keytar banjo biodiesel health goth synth bitters viral disrupt literally chia food truck.</p>
Change the text inside the <p>'s to whatever you would like and then save.