You should decimate in two steps.

[Matyooo]

One step before demodulation, decimation factor is samp_rate / amfm_bw
Other step is after demodulation, factor is amfm_bw / output_rate.
Linear interpolation should be used in second step instead of decimation

[krishnaanand1896]

Respected Sir
I would like to know whether you obtained the demodulated output.
I tried with a downloaded file FMcapture1.dat with sampling frequency 2.5 mhz centre frequency 100.122 mhz and receiver frequency 100.3 mhz.
How to modify amfm bw and decimation transition bandwidth according to the file.If you have tried with any file can you let me know which one is it.
Also your comment was to perform decimation in 2 steps,one before and one after demodulation,after demodulation,where to use this decimation,
Waiting for your valuable respinse.
Thank you