Well you could reduce all ingredients by 34% to scale the batch, but once you open extract it needs to be used asap.
If you have the boil capacity, I would brew the whole batch, ferment what you can, then put the rest in sanitary jugs and refridgerate to be reboiled in the near future.
Use the remaining wort for yeast starters or a mini batch. If the recipie has late hop additions, keep in mind they with get more bitter conversion in the reboil.
Update: A "no boil" kit shouldn't alter the IBU in the finished beer by scaling down the volumes, if done right. Most instructions have a sanitation "heating" step for the water and the extract 160-180°F. Since that heat will convert alpha acids to bitter in the beer, the hops used in the extract production have already been fully converted and can't get more bitter with more heat.
Follow the instructions. Most say to add all the extract to about 2 quarts of 160-180°F water then return to heat for 15minutes at 160-180°F. This is important for an even blend is case of any separation in the extract. This well blended mix of water and extract is what you can then use a portion of to scale down the batch size. Don't use just some of the can, it all needs to go in. Then store and save the wort concentrate as mentioned previously.