Welcome to the Forum :thumb:
3.4% seems a bit low for a kit. Most are about 4% but the best way to increase abv is to brew it a litre or 2 short. THis will not increase it by much. Adding more sugar or DME will increase abv but will affect teh taste of the brew for the worse IMO.
Alternativly buy kits with a higher abv like the Brewferm ones but these do take 3-4 months to mature :thumb: