DW1000 crystal oscillator trim

I am using the decawave dw1000 chip in one of our new designs. In place of crystal, a TCXO is used to connect to pin XTAL1. In such a case, how is it possible to trim the oscillator to reduce the initial frequency error.

You cannot trim the TCXO.

Basically you can, but not inside DW1000 IC. Some TCXOs have input trimming pin with ability to adjust the output frequency.
But I highly do not recommend doing this, typical TCXO accuracy is quite enough for almost all applications, some corrections you can made on the system level.
If you do, you would have to deal with various issues (phase noise is the obvious one). Typical adjustment range is ±10ppm, and you need to be very proficient in filtering, low-freq low-noise design etc.