1. Check the version of current Plesk control panel
2. If Plesk is of version 8.x go first upgrade plesk to 10.X then upgrade to plesk 11.0 and then go for plesk 11.5
3. If Plesk if of version 9.x then you can directly upgrade plesk to plesk 11.0 and then upgrade plesk to 11.5
Tip : Always keep a copy of the server while performing upgrade.
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