https://docs.blesta.com/display/user/Installing+Blesta#InstallingBlesta-3.ApplyHotfixFiles(ifnecessary)
If you don't have the hotfix files from when you originally set up blesta you can find them again here https://account.blesta.com/client/plugin/download_manager/client_main/index/
Back stuff up first - not my fault if you break it