Hi @hoyaniko,
You're going to want to remove the existing silicone from the gap, so you have a clean surface to work with. Put on some long protective clothing and gloves, then use a utility knife to cut out and remove the existing silicone from the drain.
Once the silicone has been removed, mask off the tiles and the drainage channel using painter's tape, then apply Light Grey Wet Area Waterproof Silicone Sealant to the gap.
You can then wipe off any excess with a damp cloth and smooth the sealant with your finger, dampened with a bit of water, then pull the tape to reveal nice, clean lines.
Check out How To Apply Bathroom Sealant for some further guidance.
Let me know if you have any further questions.
Jacob
