I live in NH and I am selling the building, I would like to inform the tenants of what's to happen in a formal letter. Is there a format I can follow or or words I should use?

There is no formal way of doing it.

Just simply tell them that you have sold the building and all of their contacts will be honoured by the new owners because they agreed, plus they have to according to the law.

Do you have a buyer? If you do, I would include in the letter an intro to the new owner and a statement that all current leases will be honored. If you just have it listed, then a mention of that fact with a statement that a condition of the sale will be that all current leases are honored.

You should do it in accordance with NH law which may include your having to advise them who the new owner is as well as their contact information AND WHO NOW HOLDS THEIR SECURITY DEPOSIT. Having 2 letters - one from you and one from new owner which acknowledges the receipt of their deposit mail together is the BEST way to do this.