You would need to reprogram it. Unfortunately this is the fallacy of having someone else do the programming for you - if it's wrong for any reason, you're stuck. However we do have several sources of information to help ...
Programming Your Uniden Scanner - The RadioReference Wiki
You will see links there for the 'Easier to Read' manuals (there's both a downloadable one and a printed one for the XT scanners you can buy from Scanner Master - either is worlds better than the garbage Uniden gave us), and our DMA FAQ which gives lots of details about how to work with DMA. There's even freeware for this radio...
Likely as not, you should ask for some help in the Texas forum (link below) - your Austin trunk is a multi site system, and there's a chance that a closer site would bring better results - although there is a snag in the fact that you have multiple sites in your system (that link is also below)...
Texas Radio Discussion Forum - The RadioReference.com Forums
Greater Austin/Travis Regional Radio System (GATRRS) Trunking System, Austin, Texas - Scanner Frequencies
If this is the system you're working with, it's possible one of the 155 mhz sites might be a better choice for you. If so, that 800 mhz antenna might not be the optimum choice. Again, someone in the Austin area is likely to be able to guide you better.
best regards..Mike