To replace the nib on your TWSBI Vac 700 fountain pen, empty and clean the pen then unscrew the front section from the barrel. Remove the cap from the canister containing the new nib and screw (hand tight only) onto the barrel before removing the canister. Replace the original nib in the canister for safe keeping.
If your pen has a coloured front section (ie not Smoke) you will need to remove the nib from the section. The nib/feed/collar assembly is a screw fit into the front section. Grip the metal ring just behind the nib and unscrew anti-clockwise from the front section of the pen. Repeat for the nib fitted to the correct colour section and swap the nibs over.