Warren sits on Route 25, roughly midway between Plymouth to the south (about 30 km) and Haverhill to the north (about 20 km). From I-93, take Exit 26 at Plymouth and follow Route 25 west through Rumney and Wentworth. The drive from Plymouth takes about 35 minutes.
There is no public transport to Warren. A car is essential. Manchester–Boston Regional Airport is about 130 km southeast; Manchester is the most convenient regional hub. Boston's Logan Airport is about 200 km south and has more flight options.
Greyhound and Concord Coach Lines serve Plymouth; from there you would need a car or a lift. Several car rental agencies operate in Plymouth and in the Lakes Region to the east.