We write the general equilibrium as:
A+BrightleftharpoonsC+D
As with any reaction there is a rate forward, "rate forward"=k_f[A][B], and of course a rate backward, "rate backward"=k_r[C][D].
Now, by definition, the chemical condition of equilibrium is defined when the forward and reverse rates are equal:
"i.e." " rate forward "=" rate backward",
"i.e." k_f[A][B]=k_r[C][D]
And upon rearrangement,
k_f/k_r=([C][D])/([A][B])
And the quotient k_f/k_r=K_c, otherwise known as the equilibrium constant for the reaction. Generally K_c is a constant for a given temperature. K_c can be large (i.e. the products are favoured at equilibrium) or small (the reactants are favoured). K_c may be formally related to the thermodynamic properties of the reaction, though I am not going to do it here.
Now K_c must be measured, and as a constant it cannot be altered. However, a chemist or engineer can certainly manipulate the equilibrium. For instance, if we remove (somehow) the products of the reaction, C and D, the equilibrium will have to re-establish itself, and it does this by moving to the right as written to satisfy the equilibrium equation, and to re-establish equilibrium concentrations of C and D. On the other hand, if we pump more reactant into the equilibrium, the equilibrium will move in a forward direction to cope with increased [A] and [B].
There should be many answers here that deal with equilibria. If there is a specific problem or query, ask, and someone will help you.
