If you're talking about a damaged internal hex bolt ?
Buy some cheap 'star-hex' bits.
Find the size that doesn't quite fit into the damaged 'hex' and tap it fully in with a hammer.
Usually works.
A lot of fixings on bikes are made of something resembling 'cheese' and require extreme care with good quality tools when undoing.
Replace damaged one with bolts with a minimum strength of 8.8 (10.9 is better) or A-2 if using stainless.