The other poster’s comment was on point, but to answer your question, you fill out a form online and they notify national grid for you. All you need is your national grid account number to make the change. Here’s the link for the municipal aggregation program, https://www.mass.gov/info-details/municipal-aggregation - select your town and then enroll in the program.

I was surprised to see the increase in my bills too, wasn’t expecting it to be so high. I’d suggest switching suppliers if your town has an agreement in place as it tends to be the cheapest and same pricing for the length of the contract, it’s really easy to do online. It’s still going to be billed through national grid so not sure that building management will need to know. It takes about a billing period to take effect, but will save you money for a bit.