Hi,
Here's a teardown [https://www.youtube.com/watch?v=FiT2Jnv18Ik|video] that shows the removal of the screen so that it can be replaced.
Here's just one [https://www.ebay.com/itm/144359583685|supplier] of the screen. (example only to show cost of replacement part)
There are other suppliers online that may suit you better. Just search for ''Xiaomi Mi band 6 LCD screen'' to get results.
If doing this yourself seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote.