You can install the plugin in two different ways:
Manual installation via FTP (RECOMMENDED)
Go to System → Cache management and deactivate all Magento caches.
Select all.
Deactivate.
Send.
Decompress the file Sequra-Sequrapayment-X.Y.Z.tgz
Upload via FTP the Magento installation directories to the server. They won’t be overwritten despite already existing those directories.
Refresh Magento’s cache in System - Cache management
Select all.
Refresh.
Send.
Once you have set up the plugin you can reactivate the caches.
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Using Magento Connect
You can also install the .tgz file via Magento Connect.
...
Access to System → Magento Connect → Magento Connect Management
Introduce the access credentials into Magento management.
Upload the file Sequra-Sequrapayment-X.Y.Z.tgz in the 3rd block, Direct package file upload.