Q2: How to replace the soldering tips when the tips doesn't work?
Soldering tips have to be kept clean not just of old solder and flux build up, but of oxidization as well. The base metal of all soldering tips is copper, and copper exposed to air will oxidize. This is why re-tinning a tip is a necessary part of cleaning and caring for it. Re-tinning puts a protective coating on the copper that prevents oxidation. Weller tips are made in such a way that they need to be cleaned and tinned with Weller products only to keep them in good working order. The older technique of cleaning a soldering tip with a wet sponge will harm a Weller tip. 

1) Unplug the soldering iron and allow it to cool completely.

2) Unscrew the locking nut that holds the tip-retaining sleeve and tip in place. If the tip is interchanged with different sizes frequently, this may be done by hand. Utilize a set of small pliers to free the locking nut if frozen.

3) Slide the nut and retaining sleeve off the soldering iron heating element and remove the tip from the sleeve.

4) Place the desired tip into the retaining sleeve and slide it back onto the soldering iron heating element. Slide the retaining nut over the tip and retaining sleeve and hand tighten.

Note: Always unplug the soldering iron and allow it to cool completely to touch before attempting to change tips. Serious burns could result.

Q3: Which type of soldering tips will be used with Hakko soldering station?

1) Hakko 900 series of soldering tips will be used with Hakko 936, 937 soldering station and other equivalent soldering stations.

2) Hakko T12/T15 series of soldering tips will be used with Hakko 951 soldering station and other equivalent soldering stations.