To update your Seafile Client (e.g. 3.1.7) on your Raspberry you just have to install the new packages of version 4.x.

1. Download packages

• libsearpc-3.0_3.0.4-1_armhf.deb (Same as in Version 3. Not update needed)
• ccnet_4.0.6-1_armhf.deb
• seafile_4.0.6-1_armhf.deb
• seafile-client_4.0.6-1_armhf.deb

2. Install packages

dpkg -i libsearpc-3.0_3.0.4-1_armhf.deb
dpkg -i ccnet_4.0.6-1_armhf.deb
dpkg -i seafile_4.0.6-1_armhf.deb
dpkg -i seafile-client_4.0.6-1_armhf.deb

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