If you'd like to raise innodb_buffer_pool_size without restarting mysqld, you can set the variable at run-time. The size unit is in byte.
mysql> set global innodb_buffer_pool_size = 5368709120;
Query OK, 0 rows affected (0.01 sec)
mysql> show variables like 'innodb_buffer_pool_size';
+-------------------------+------------+
| Variable_name | Value |
+-------------------------+------------+
| innodb_buffer_pool_size | 5368709120 |
+-------------------------+------------+
1 row in set (0.00 sec)
Please note that, you can't set the parameter to a value that is less than the current one at run-time.
For permanent setting, you can add the key-value pair under [mysqld] section in /etc/my.cnf.