From what you have written I expect you know the rule that says that when the direct object is a pronoun, it must be placed before the verb.

J’améliore la peinture → Je l’améliore.

I suppose you hesitate because there are two verbs in your sentence and you do not know in front of which verb (vouloir or améliorer) to place the pronoun.

La peinture is the direct object of améliorer, so when you replace la peinture by a pronoun you put it directly before améliorer.

Je voulais améliorer la peinture. → Je voulais l’améliorer.

Note that in your first sentence le is wrong because peinture is feminine.

La peinture, je voulais la changer.