We can use ROW_NUMBER along with a calculation to find the first of the month:

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY CONVERT(VARCHAR(7), SnapahotDate, 120)
                                 ORDER BY SnapshotDate DESC) rn
    FROM yourTable
)

SELECT DATEADD(month, DATEDIFF(month, 0, SnapshotDate), 0) AS SnapshotDate,
       NrAccounts,
       Status,
       AccountType,
       RegisteredCountry,
       RegisteredCountryID
FROM cte
WHERE rn = 1
ORDER BY SnapshotDate;

You can use window function and Cte to solve the problem

you use FORMAT to find part YYYY MM of date

;WITH _Lstdate AS (
    SELECT 
              SnapshotDate
             ,NrAccounts
             ,Status
             ,AccountType
             ,RegisteredCountry
             ,RegisteredCountryID
             ,  Rank() OVER (
                                  PARTITION BY  FORMAT (SnapshotDate, 'YYYY-MM')
                                  ORDER BY SnapshotDate desc
                             ) rw
    FROM Lstdate
)

SELECT 
       FORMAT (SnapshotDate, 'yyyy-MM')  +'-01'  AS SnapshotDate,
       NrAccounts,
       Status,
       AccountType,
       RegisteredCountry,
       RegisteredCountryID

FROM _Lstdate
WHERE rw = 1
ORDER BY SnapshotDate

You can insert the basic data with the following statements:

drop table if exists Lstdate

create table Lstdate (SnapshotDate date,NrAccounts int,Status varchar(100),RegisteredCountry varchar(100)
,RegisteredCountryId int,AccountType varchar(100))

insert into Lstdate
(SnapshotDate,AccountType,NrAccounts,Status,RegisteredCountry,RegisteredCountryId)
          select '2023/01/01' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,585  as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/02/01' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,585  as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/03/01' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,585  as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/03/29' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,585  as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/04/11' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,7578 as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/04/13' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,58   as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/04/14' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,58   as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId
union all select '2023/04/15' as SnapshotDate ,'BELGIAN_NRN' as  AccountType,22   as NrAccounts,'ACTIVATED' as Status,'Belgian' as RegisteredCountry,1 as RegisteredCountryId