Without knowing the make and model of your shower mixer, precise advise is not possible. But what you should be able to do, is remove the temperature control knob to expose the mechanism it attaches to. You may find a plastic ring which stops the knob turning any further to avoid very hot water coming out. You should be able to adjust this stop to increase the temperature of the shower water. Trail and error will get you the correct result in the end.
The leaking shower head would probably best be solved by buying a new one, or at least look at the washer where the hose is connected. The leaking shower head will not be the cause of the temperature issue.
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