In the Nordic Semiconductor SDK examples, the BLE connection parameters are typically in main.c toward the top of the file:
#define MIN_CONN_INTERVAL MSEC_TO_UNITS(400, UNIT_1_25_MS) /**< Minimum acceptable connection interval (0.4 seconds). */These will need to be changed to the desired connection interval (7.5ms min), though they are subject to negotiation by the central. For the connection interval, iOS tends to allow 20-30ms minimum and Android a little faster.
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(650, UNIT_1_25_MS) /**< Maximum acceptable connection interval (0.65 second). */
#define SLAVE_LATENCY 0 /**< Slave latency. */
#define CONN_SUP_TIMEOUT MSEC_TO_UNITS(4000, UNIT_10_MS) /**< Connection supervisory timeout (4 seconds). */