Here's a video on soldering type slugs: Typewriter Soldering QWERTY Swap- Shim Method- Step by Step Tutorial (youtube.com)

Here's Mary at myoldtypweriter doing some soldering: Old Dog, New Tricks – myoldtypewriter

And is it possible to only use tin for soldering ?
Because lead is toxic...
 

The problem with silver solder is that the joint has to be heated to red heat, thus destroying the temper of the spring steel typebar and potentially weakening it.  If the type slug has slipped slightly during the process, it would be difficult to undo the joint and do it all over again.  A soft-soldered joint is slightly elastic, thus acting as a shock absorber when the typeface hits the paper.  A hard-soldered (silver solder joint) is not.  Possibly you could use lead-free solder (tin), but I do not know if it has the same elasticity.  Also, it does not have the same plasticity/temperature range as lead-based solder so may be more difficult to work with in this application.  Finally, regarding lead being a poisonous metal - yes it is, but in the very small amounts being used it is hardly likely to cause you any problems.  After all, until about ten years ago all the copper drinking water pipes in British homes were soldered together with tin/lead, and most still are.  I don't think anyone ever died from it !

The correct material is 'tinmans solder', which is 40% tin, 60% lead.  However, ordinary electronic solder, which is 60% tin, 40% lead will work just as well in practice.