Removing some of the solvent (usually by evaporation) will leave solutes behind in a relatively larger ratio to the solvent. Taken to the extreme, the solvent can be eventually boiled/evaporated off entirely, leaving only the solutes.
This only works when the solution is one of a dissolved solid in a solvent. Miscible liquids do not work as well, although it is the mechanism of fractional distillation.
Alternatively, for both liquid and solid/liquid solutions, simply adding more of the solute will increase the overall solution concentration of that solute.