you can upgrade to any version of JIRA released till 12 months from the date of purchase
http://www.atlassian.com/licensing/purchase-licensing
check the comments out at this answer:
hi,
if you mean upgrade the user tier; you can enter your sen id and mail address that you purchased the license here, find out about your license and upgrade it:
https://www.atlassian.com/purchase/upgrade
if you want to update the JIRA version, you can find them here:
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