Spread bets and CFDs are complex instruments and come with a high risk of losing money rapidly due to leverage. 76% of retail investor accounts lose money when trading spread bets and CFDs with this provider. You should consider whether you understand how spread bets and CFDs work and whether you can afford to take the high risk of losing your money.
To close a position partially, please go to Account -> My accounts -> Trade options -> select Normal mode. After that you need to open a trade in an opposite direction for an appropriate size and the existing position will be closed partially.