In case you are planning to upgrade your old phone and planning to get a Xiaomi device the you can make use of Mi Exchange Program. The old device that you are using should be in the list that can be exchanged.
The process is simple all you need to do is go to the Mi Exchange page here. Then look for the device you are using, if it is listed in the devices list (there are many and from 15 brands). If the device is listed there then it can be exchanged, however there are some criteria.
The device that you own and wish to exchange should be in working condition, the physical damage on the phone should be normal wear and tear and not something that is caused by the user, for example there should not be any cracks, body dents, screen cracks etc.
Once you select the device you will get the price that you can get for your old device. You also need to provide the IMEI numbers of your old phone. If you wish to go ahead then accept the terms / conditions and get exchange coupon.
Once done you can order a new phone from Xiaomi and keep the old phone ready it will be picked up at the time of delivery. Do check the terms and conditions and also check the price you get for the old handset, if good enough go ahead.