I assume you are one of the parties to the stipulation you'd like to have extended, and that you are able to contact the other party, or its attorney.
I'm tempted to answer "Very, very politely," but I don't think this is helpful and may be entirely obvious.
A one-sentence question like this doesn't give the LawGuru volunteers much to work from; we know nothing about the reason for the stipulation in the first place, or why the time originally allowed turned out to be insufficient. Most stipulations of this sort are intended to give a party time to carry out a responsibility. If there are good reasons, they should be raised with the other party. One can also mention abstract notions like "professional courtesy" or "good-faith efforts."
Most parties will extend stipulations, within reason, if they can see genuine progress being made and are convinced that an extension is warranted and reasonable under the circumstances. The extension should be negotiated before hand by telephone, e-mail, or in-person meetings, then put in writing.
