To update your Seafile Client (e.g. 3.1.7) on your Raspberry you just have to install the new packages of version 5.x.

1. Download packages

• libsearpc-3.0_3.0-latest-2_armhf
• ccnet_5.0.1-1_armhf
• seafile_5.0.1-1_armhf
• seafile-client_5.0.1-1_armhf

2. Install packages

dpkg -i libsearpc-3.0_3.0-latest-2_armhf.deb
dpkg -i ccnet_5.0.1-1_armhf.deb
dpkg -i seafile_5.0.1-1_armhf.deb
dpkg -i seafile-client_5.0.1-1_armhf.deb

As usual: Please, report if you have any issues and also report if it worked for you.