Generally, you can re-mortgage your home at any time.
However, you should bear in mind that If you are on a fixed rate deal and your current product is not due to expire in the coming months or years, a re-mortgage before this comes to an end may incur early redemption penalties which will be payable to your existing mortgage lender.
These tend to be a percentage of the total outstanding loan, so can be quite large in some instances. It is always best to time a re-mortgage to fall in the window where you product is expiring (if this is possible). There may be circumstances where waiting until the end of the existing deal may not be an option.