How to drop a unique constraint from table column?

ALTER TABLE users
DROP CONSTRAINT 'constraints_name'

if earlier constraints_name is not provided, it will have some default constraint_name, in pgAdmin 4 (pSql), try violating the constraint and you can see the constraint_name being violated in the error received, most probably same must be the case with other platforms or there are some articles available over web where constraint_name is extracted from certain tables where they are stored, not sure about this though. P.S : Can take reference from comments also

This works mostly.

drop index IX_dbo_YourTableName__YourColumnName on dbo.YourTableName
GO

SKINDER, your code does not use column name. Correct script is:

declare @table_name nvarchar(256)  
declare @col_name nvarchar(256)  
declare @Command  nvarchar(1000)  

set @table_name = N'users'
set @col_name = N'login'

select @Command = 'ALTER TABLE ' + @table_name + ' drop constraint ' + d.name
    from sys.tables t 
    join sys.indexes d on d.object_id = t.object_id  and d.type=2 and d.is_unique=1
    join sys.index_columns ic on d.index_id=ic.index_id and ic.object_id=t.object_id
    join sys.columns c on ic.column_id = c.column_id  and c.object_id=t.object_id
    where t.name = @table_name and c.name=@col_name

print @Command

--execute (@Command)