How to set the 2M PHY Mode while searching for a Bluetooth device?

The PHY mode is negotiated between the two connected devices, after the connection is established. This is based on the capabilities of both sides, as well as properties of the connection. This is a link layer negotiation, and as such, apps have no control over how the connection is established. If you are the manufacturer of the accessory that is connecting to an Apple device via an app, it would make the PHY update request after the connection is established.

If I were to read your question literally about setting a PHY mode while searching; If by this you mean you want to scan and discover only devices that will use a particular PHY mode, that does not make much sense, as the final PHY mode can only be known after the device has been discovered and connected to.

Is it possible to set Long /Short mode connection after established? we need some capability to change the default of Blettoth 5.2 Long Mode to short mode so that we can achieve better data thought put to speed.

my original problem is as described below the sensor.

we are using an external sensor to connect and get the accelerometer data. with New Firmware it's about 50% more time than with old Firmware .and approached the vendor he was saying ***** (sensor) is able to capable what Master (IOS iPhone device) and he said he unable to find to set PHY mode from Apple Core Bluetooth SDK. for brief understanding I have added the link of communication below.

https://mbientlab.com/community/discussion/4076/observed-the-change-in-download-recorded-data-from-sensor-to-phone#latest