A Re-entry Permit cannot be extended or renewed. Generally, it is valid for 2 years from the date of issuance, but the validity period may vary. When your Re-entry Permit expires, you need to apply for a new one by filing Form I-131, Application for Travel Document, again.
To receive a new Re-entry Permit you will additionally have to indicate in Form I-131 information regarding the disposition of your previous travel document (whether it is expired, lost, stolen, destroyed, etc.). Please find the details concerning disposition in our article “What is “disposition” on Form I-131”.
And if due to some reasons you need to travel outside the USA on regular basis, consider consulting with an immigration attorney.