Yes, you can upgrade to the current 6-series.
We generally advise first upgrading to the last 5-series version, 5.47e, if you were using a version prior to 5.47 (such as your 5.45).
5.47e is still available in the Member Area, so you can grab it when you renew your license.
You should review the upgrade docs for more information:
http://www.infopop.com/support/ubbclassic/UBB6_upgrade_5_to_6.html --
Charles Capps
UBB.classic™ Developer
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