Log_archive_format

It's an init paramter.

The docs.  When are you going to go to the docs first:
http://download.oracle.com/docs/cd/E11882_01/server.112/e17110/initparams124.htm#REFRN10089

where shall we chnage "Log_archive_format=%t_%s_%r.dbf" to Log_archive_format="dbname_%t_%s_%r.arc" on linux servers for oracle 11g database, I mean in pfile or we can use
SQL> alter system set Log_archive_format="dbname_%t_%s_%r.arc";

If I am wrong please correct me. I really appreciate your help.

For most init parameters you can use either option:
1. Change the value in the pfile (if you use a pfile)
2. Use the "alter system" command to change the current value in memory and/or change the value in the spfile.

If you use the pfile option, those changes will not take effect until you stop and restart the database.  If you use the "alter system" option (this assumes you use an spfile, not the older-style pfil), the change will take effect immediately (if this is a parameter that supports dynamic changes).  

But, this particular parameter: log_archive_format cannot be changed dynamically.  So, if you use a pfile, you can make the change there, then stop and restart the database.  If you use an spfile, you can use an "alter system" command, but you must include "scope=spfile", since the change can only take effect the next time the database is restarted.