Either use the mongo command line or RoboMongo or MongoChef. Don't forget that you don't necessarily have to open 27017 on the network to use it (ssh tunnel & use localhost:27017).

See what databases are available and choose one,

show databases
use <database name>

Searching Data

Search for a document containing foo in the users collection,

db.users.find({username: 'foo'});

and // makes it case-insensitive,

db.users.find({username: /foo/});

Updating Data

Reset the value for e.g. happyTimeout in the users collection on all the documents (multi true),

db.users.update({}, {$set: {happyTimeout : null}}, {multi : true})

Dropping a Database

Check the db names,

show dbs

Copy & nuke a DB entirely,

db.copyDatabase('dbname', 'dbcopy');
use dbname

or as one-liner from the shell,

mongo dbname --eval "db.dropDatabase();"