Labor unions can increase wages through:
collective bargaining
Reducing supply of labor
and increasing demand for labor
In collective bargaining, the workers can send their representatives to negotiate new working terms with employers (including work hours and increasing wages).
Reducing supply of labor would make the existing laborers become more lucrative, rare and valued. This would resulted in an increase of workers' wages.
Increasing demand for labor can be done by improving the members' skills through courses or seminar. By having additional skills, the members can be eligible to harder and more rare job opportunities. This usually correlate with higher wages