Reputation in Fallout: New Vegas is tracked per faction as two numbers — fame and infamy. The console can set or add either one, letting you repair a ruined standing with the NCR, the Legion, or any town. You'll need the faction's reputation ID from the faction reputation IDs list.
Set reputation — setreputation
Open the console with ~ (how to open the console), then use setreputation <faction id> <0/1> <amount>. The middle value is the type — 1 for fame, 0 for infamy:
That sets your fame with the Boomers to 50. Set infamy to 0 to wipe a bad reputation:
Add reputation — addreputation
To nudge a value instead of setting it, use addreputation with the same arguments:
Find faction IDs
The reputation ID for every faction (NCR, Caesar's Legion, Goodsprings, the Strip…) is on the faction reputation list, each with a copyable setreputation command.