How to collect Shimano ALTUS parts together?
Shifters are notoriously hard to reassemble, and if it came to pieces unexpectedly it's going to be hard to know whether you've got everything. You also don't know (until you've put all the effort in to try and fix it) whether the reason it gave you trouble in the first place is something you can fix. An Altus shifter shouldn't be expensive so it's probably best to replace it.