this is a common problem we are facing now a days. All we can do is to wash it carefully. For more we can follow some basic steps mentioned below:
Step 1

Your wool or 100% cotton shirt needs to be washed for one cycle at the highest temperature setting on your washing machine. Stretched fabric fibres can be helped to contract back to their original size by the interaction of moisture, heat, and agitation.

Step 2

The shirt should be taken out of the washer and put in the dryer. Till the shirt is entirely dry, run a drying cycle on medium heat. For a more noticeable shrinking impact, turn the dryer on high heat, unless the shirt's label specifically advises against it.

Step 3

Up till the shirt is snug enough for you, repeat the washing and drying process. The initial washing usually results in the biggest shrinkage. Even though the benefits of further washings will lessen, the shirt may still slightly shrink.