Complex sentence: She was paid off in 1815 only to be recommissioned in 1817 when she was modified to carry 76 guns when her stern was altered and diagonal bracing on the framing introduced by Sir Robert Seppings.
Split into simpler sentences: She was paid off in 1815 only to be recommissioned in 1817 when she was modified to carry 76 guns. At the same time her stern was altered and she was given diagonal bracing on the framing introduced by Sir Robert Seppings.