You can't! It's hard-coded into the source code. And
unless you are willing to spend the next 5+ (possibly
more I just don't know the full penalties for this) years
in jail for cracking/reverse-engineering the app, then
you will have to live with it. Plus it will probably
take you several years to crack/reverse-engineer the
source code.
Beta products are almost always only for a limited-time.
The current plan is to release Beta 2 sometime later this
year. If that doesn't come to fruition, then they will
likely release a refresh update like they did in July to
push the expiration date back. Before they released that
update the product was supposed to expire on July 31.
Alan