3 Could we change the time?
我们能不能换个时间？
Could we change the time? I'd like to go over some issues before the meeting. Do you think you can come an hour early?
我们能不能换个时间？我想在会前处理一些事情。您能否提前一小时过来？
Could we have the meeting at 1 o'clock instead of 2?

我们能把会议时间从两点改到一点吗？

4 Let me know when is convenient for you.
请告诉我您何时方便.
I think it is better if we could discuss it in person rather than via e-mail. Let me know when is convenient for you and we'll schedule a meeting.
我认为当面泰伦该问题比发送电子邮件更好。请告诉我您何时方便，以便我们约定见面的时间。