Remove the identity spec for the column & calculate & insert the value yourself
You can get the highest Id using
Select MAX(Lab_Id) from patient
You can increment & insert the new value
If the year has changed, calculate the new id like
int Id = ((DateTime.Today.Year % 100) * 1000000) + 1
============================================
Alternatively, you can still use auto increment & choose to temporarily turn off auto increment and insert value when the year has changed.
You can use
SET IDENTITY_INSERT patient ON
After this, you can insert custom id value and turn off using
SET IDENTITY_INSERT patient OFF
Remember that IDENTITY_INSERT can be turned on for only one table at a time.