you’ll need a pair of long nose pointy pliers.
Unscrew the end cap (with the clicky switch in it) using the pliers unscrew all the internals, one by one. Clean all the threads and lightly scour the brass battery contact.
Srew the internals back in in reverse order making sure there all good and tight.
Should sort it, my mates is always going off/dim and it always reponds to the above.