The simplest way to close a position partially is to open a new position in an opposite direction on required quantity.
In an example below, client is holding a long position, 10 CFDs of Germany 30 and wish to reduce to 5.
He opens a trade ticket and sells 5 CFDs of Germany 30 at market (it is also possible to choose a different order type).
Now client has only 5 CFDs of Germany 30 in his portfolio.
As you can see in above screenshot, both 10 CFDs long and 5 CFD short are showing in Positions module, with net long position of 5 CFDs.