Yes you can reheat lemon curd to thicken it.
The way I've thickened lemon curd is to place the lemon curd into a saucepan and then turn the stove on low heat and then stir the lemon curd until it's thickened to the thickness I want it.
Then let it cool down before serving it but you can eat it served warm or cold it depends on how you like it but lemon curd is usually served cold.
I like my lemon curd served at room temperature but some people prefer to have their lemon curd served chilled.
But I've always had good luck by heating the lemon curd up to thicken it when it's turned out too thin.
Usually now though I get it thick enough the first try but at first I made the lemon curd to thin and just reheated it and it thickened up nicely.