To reduce the error and uncertainity in the measurement of the period of the pendulum, you have to determine the period repeatedly. After getting a number of values for the period, take the average of those values. This average will have minimum error and uncertainity.
To reduce the error in the measurement of the length, you can follow the same procedure.
Or you can use the value of the other physical quantities which is the average of the repeatedly measured values to calculate the length of the pendulum.
Therefore to minimize the error and uncertainity in any values, you can take the average of the repeatedly measured values or you can use the average value of other physical quantities to calculate the value of the physical quantity you want.