Changes of Notice Period

We are looking into revising employees' notice period from 1 month to 2 months.

If this is the only revision to their employment, is it necessary to issue a new contract ?  Or just need to issue a letter stating this and obtain their acceptance (by signing and returning the letter) ? Under what circumstances do we require to issue revised contract  or offer letter ?

You need to issue letter and with their agreement, new LOA is not required unless changes of other terms and condition in the contract