Rather than setting a fixed rent for the entire term of the lease, most leases (particularly longer leases) include rent reviews which allow the rent to be adjusted periodically. The terms of the lease will set out how the rent reviews work, including when rent reviews will take place (eg every three years) and how the new rent will be calculated. The lease will usually also specify what notice the landlord must give you of the new rent and how disagreements will be resolved.
Most leases base the new rent on the ‘open market rental value’ at the date of the rent review – ie the rent the landlord could reasonably expect to receive if the premises were leased on that date to a third party, on similar terms to those in your lease. Some leases link the new rent instead to the Retail Price Index.