There are two types of pens that have replaceable nibs: fountain pens that take ink and ink cartridges, and electronic tablet pens that write by transmitting data via the electronic nib, rather than with physical ink. Both are easy to replace although each set of instructions is different.
Dab any ink residue from your fountain pen with the paper towel.
Grip the fountain pen nib between your thumb and forefinger and pull firmly to remove the existing nib. Rinse it with water before storing it or putting it on a different pen.
Holding the new pen nib between thumb and forefinger, press it firmly into the top of the pen at the same angle and orientation as the old nib.
Hold the tablet pen in one hand and use the tweezers to grip the nib. Pull firmly in one swift stroke to remove the nib. If the nib is damaged or flattened, throw it away as it will not be usable.
Insert the new nib, several of which usually come with a tablet pen, into the pen either with your fingers or the tweezers. It will be longer than an inch, so keep pushing until you meet resistance. If there is more than a quarter-inch sticking out the end of the tablet pen, shake the pen gently and push again until the nib pops in.
Use the tablet pen gently to keep from damaging the new nib.