Estimate the interest saved and the shorter tenure (or lower monthly payment) from a lump-sum partial repayment.
Prepaying reduces your outstanding principal, so less interest accrues. You can either shorten the tenure (keep the same monthly payment, finish earlier) or lower the monthly payment (keep the same tenure). For the same lump sum, shortening the tenure usually saves more interest.
Prepayment is most effective early in the loan, when the interest portion of each instalment is largest. Watch for lock-in periods and prepayment penalties, and keep enough emergency savings. If your mortgage rate is higher than what you could safely earn elsewhere, prepaying is often worthwhile.