I've seen this play out a few times with people I know, and it almost always comes down to what the actual lease document says. If your lease doesn't have a clause allowing mid-term increases, your landlord can't raise the rent until March 2027 when you're up for renewal. The lease is a binding contract - both parties agreed to those terms, and inflation isn't a legal reason to break that agreement mid-term. Your landlord is essentially trying to renegotiate without your consent, which doesn't work that way.
Here's the thing though, and this is where people sometimes slip up: don't just assume the lease prohibits it. Pull out that document and read it carefully. Some leases have escalation clauses that allow increases every year or every few years, and they might be buried in there in language like "annual adjustment" or "cost-of-living increase." If your lease has something like that, your landlord might actually have legal ground. Alternatively, a few jurisdictions have weird rules around what constitutes a valid lease term, so knowing your state or local laws matters. That said, most standard residential leases don't allow this, and if yours doesn't have explicit language permitting it, you're protected until renewal.
If the letter isn't pulling from actual lease language, respond in writing (keep it simple) saying the rent is locked until March 2027 per the lease terms. Don't agree to anything or acknowledge the increase. If they keep pushing, that's when you'd want to look at your specific location's tenant protections, since some areas have additional rules your landlord might be violating.