Reducing the tolerance of resistors manually
Tolerance is a statistical abstraction which tells us how much variation to expect from a sample of resistors of a given type. A single resistor doesn't have tolerance: it has a value, which deviates to some extent from the stated value (and of course varies with temperature).  We cannot change (manually or otherwise) a property that a part doesn't have.

If part of a circuit requires a very accurately set resistance (which perhaps has to compensate for variations in other parts, and so cannot be a fixed value, no matter how precise), we can use a potentiometer.  A potentiometer or rheostat is a resistor whose value you can manually change.

A useful trick is to use a fixed resistor for most of the resistance, and a smaller-valued potentiometer just for the adjustment. For instance, if we wanted to adjust 100K within a +/- 5% range, we could get a 95.3K resistor (E48 series number), and a 10K potentiometer wired as a rheostat in series with that resistor.