General Synonyms:
* Gave in
* Yielded
* Submitted
* Complied
* Acceded
* Agreed
* Consented
* Gave way
* Caved in
* Buckled
* Softened
* Relinquished
* Let up
Synonyms emphasizing change of heart:
* Changed one's mind
* Had a change of heart
* Became more lenient
* Showed mercy
* Became compassionate
* Took pity
Synonyms emphasizing a softening of stance:
* Relaxed
* Loosened
* Eased up
* Became more flexible
* Became more understanding
The best synonym to use will depend on the specific context in which you're using the word.
For example, if someone is "relented" in their opposition to a plan, you might use "gave in" or "yielded." If someone is "relented" in their anger, you might use "softened" or "had a change of heart."