You have to login to application node as the owner of the EBS and source (run) the environment file. Then, you can run FNDCPASS from any location.
FNDCPASS is the utility used in EBS to change such user's passwords. e.g, SYSADMIN,APPS
It cannot be used as ROOT User. You have to login into EBS application node as a owner of that node. such as applmgr user. Then navigate to $APPL_TOP and load your application's environment file.
Also, there is no such specific location which is being used to run this FNDCPASS utility. You can run it from anywhere from PUTTY.
I hope this answers your question in some more details.