As trademarks are normally valid for 10 years trademark renewal is the opportunity to extend the lifespan of thtrademark with a renew. Once a trademark is renewed, a certificate of renewal is issued to the trademark owner. Trademarks can be renewed repeatedly without limitation, as long as normal criteria such as contnued use are met.
Trademark renewals vary between registries, but most require renewal 10 years from date of registration (or last renewal date). Renewal periods differ between registries. Canada is a notable exception where trademark registration lasts for 15 years.
There are similar requirements for renewals at most registries. A renewal form must be completed with details of the registered mark that’s being renewed. Some registries require supporting documentation to cover evidence of use and other such formalities. Fees are also payable, and these increase where the renewal is late (of the registry allows late renewals).
No, you cannot make changes to you trademark when you renew it.
There is no limit o the number of times a trademark can be renewed, as long as renewal requirmeents are met.