Old thumb shifter slips - can it be mended?
These shifters are not really supposed to be repaired. Sometimes contamination interferes with the mechanism and can be cleaned out, but if the ratchets have worn out you just need to replace them.

Shimano makes 8 speed shifters that are compatible with older derailleurs. The current Acera/Altus range includes 3x8 and 2x8 shifters for 2 and 3 front chainrings.

Here's the Shimano range line-up page for Acera/Altus: https://productinfo.shimano.com/#/lc/acera_altus/3x8