Rocket engines can certainly be used to increase and decrease the speed of the spacecraft. However, rocket thrust takes propellant, propellant has mass, and even a small change in velocity (known as Δv, or "delta-v", the delta symbol being used to represent a change and "v" signifying velocity) translates to a far larger requirement for propellant needed to escape Earth's gravity well. This is because not only must the primary-stage engines lift the extra propellant, they must also lift the extra propellant beyond that which is needed to lift that additional propellant. The liftoff mass requirement increases exponentially with an increase in the required delta-v of the spacecraft.
rocket engines can certainly be used to increase and decrease the speed of the spacecraft however rocket thrust takes propellant propellant has mass and even a small change in velocity (known as δv or "delta-v" the delta symbol being used to represent a change and "v" signifying velocity) translates to a far larger requirement for propellant needed to escape earth's gravity well this is because not only must the primary-stage engines lift the extra propellant they must also lift the extra propellant beyond that which is needed to lift that additional propellant the liftoff mass requirement increases exponentially with an increase in the required delta-v of the spacecraft