The biological impact of electromagnetic radiation from mobile phones has still not been fully researched. Until further investigations have been carried out, it makes sense to reduce the amount of time you spend on your phone.
Newer phones tend to be safer, but this is not a sufficient reason to replace your old device. In the interests of environmental protection, you should think carefully before discarding an old mobile. If you really do need a replacement, and are not simply following the latest fashion, then make sure you recycle your old phone.
You should never discard a telephone with the battery still inside. Because of the chemicals it contains, the battery must be removed and taken to an appropriate disposal site, where it can be recycled separately.